You can try wedging a sharp Torx driver of the appropriate size in there and seeing if it will grip. Failing that, I have left-hand drill bits for difficult cases, but that takes some care.
Funny, that is almost precisely what I was going to reply with.
I had good results with this on a Tek 7D20.
Hex set screw was damaged from previous whatever (eBay purchased). Completely stuck and hex driver would then slip.
Tried a Torx but it is really seized.
Made a thin plastic square with a slot to the middle to clear the shaft and holding nut to insert under the knob and protect the faceplate in case I have to go medieval on it.
Orient the plugin so that I can let some "Liquid Wrench" in the hole overnight, repeat the day after. No dice.
Used a generous dose of "Super Cold" spay with a thin tube in the hole and waited for the whole area to freeze over. After about 5 minutes used a hair drier to warm the assembly quickly.
On the second trial the Wera Torx got a good grip and got it. No damage to the front plate either, only a new set screw.
Good luck with it, hope this helps.
I have had good success with a couple of drops of "Weasel Piss" on stubborn set screws - works better than Liquid Wrench and similar products.
Be careful about spilling this on plastic though.
I have pictures but forum is not allowing me to upload them. Weird!
I have the same problem with the forum all the time now.
I've had this problem a few times too. There is one thing to try before doing anything that will further round out the allen key hole.
This trick comes from realizing that the grub screw isn't actually 'seized' (ie corroded in, where corrosion products have greater volume than the original metal, so the threads are seriously locked in.) Nor is it spalled (metal graunched off the threads, making a jam.)
It's more likely just very tightly screwed in, ie there's a force where the grub screw tip presses into the shaft, and an equal force spread out over one side of the threads in the knob. That's ONE side of the thread, not both. Hence if the shaft could be magically removed (so no force anymore) the grub screw would turn freely, even with a rounded out allen key hole.
But the thing is, you don't have to remove the shaft _entirely_. You just have to make it not quite so large, right where the grub screw touches it.
And that is easy!
For this you need a small pin punch (flat end), that fits in the grub screw hole. Best if it is a close fit, so the end rests on the top of the grub screw. Then arrange some side support for the knob, that can take a strong force without damaging the plastic. Scrap piece of wood, use a hole saw to cut a hole that pretty closely fits the circumference of the knob. Cut one side off so you have access to the grub screw hole. Get this support resting against something solid, or in a vice, take up shaft sideways slack so the bearing sleeve doesn't take the impact...
Now whack the grub screw downwards into the hole, with a sharp impact from a small hammer on the pin punch.
The aim is to drive the grubscrew end slightly into the shaft, deforming the shaft metal (typically brass) just a little bit. This relieves the screw pressure. It will also break any adhesion between the grubscrew and the knob threads.
It's surprising how small an impact will work.
Now the grub screw will be loose, and easily backed off enough to remove the knob.
If you still have trouble turning it enough to remove the damaged grub screw entirely, once the knob is off you can turn the grub screw *inwards* more easily, till it falls out the central hole.
Remember to make sure whatever you insert into the allen hole is FULLy BOTTOMED out.
Make use of the Russian precision tool (hammer) with support on the other side of the knob to ensure all is well.
The hammer needs to be very light for this to work, it seems to me. Sounds a good idea.
Another trick that can work in some situations, jam a torx bit or even a hex key in there, then fill the gaps with baking soda and use a syringe to dribble a bit of cyanoacrylate (super glue) in there to saturate the baking soda. It cures quickly into an incredibly hard substance, a trick I first learned to use as a filler while building model airplanes I've since used it in a handful of other applications. Of course you have to be very careful not to get any of the glue on the threads of the thing you're trying to get out, so best not to fill it all the way up to the brim.
Remember to make sure whatever you insert into the allen hole is FULLy BOTTOMED out.
Make use of the Russian precision tool (hammer) with support on the other side of the knob to ensure all is well.
"a small pin punch (flat end), that fits in the grub screw hole" - Actually I worded this poorly. I meant 'fits in the threaded hole in the knob'. The pin punch should sit on the top of the grub screw. The grub screw should be hardened steel, and should not deform.
You'd be hard pressed to find a pin punch small enough to fit down the hex socket on the grub screw, and if it did then it would probably be too thin to transmit the required force.
Re hammer size: I use a small tack hammer, with quite a sharp, fast hit. You only need to make a tiny deeper dent in the brass shaft where the grub screw is already dinked into it.
The punch method makes sense, I'll have to try that one next time I have a stubborn grub screw.
Really hate those things.
I think the best way to get it loose is with heat as previously mentioned.
Take a tool, either hex or torx that fits snuggly in it, cut the 'L' part off and clamp it in a high wattage soldering iron. It doesn't need to be perfectly centered in the soldering iron but the tool needs to be clamped tightly in it. I had an older, high wattage iron with a large set screw in it for holding the tip. Once it gets hot, you might be able to twist it out with the bit in the soldering iron, should be able to at least break it loose. But if you heat it up it should come loose more easily with a modified easyout or another wrench.
Prior to doing this, apply aluminum tape they use on HVAC tubing to protect all the plastic parts. If you have room, you might be able to stick a small tube cutoff down in the hole to protect the plastic around the hole in the knob.