Nothing except too much cable, which I can chop and attach my own micro plug at the exact position I want.
Just interested if anyone had experience of alternatives.
Thanks.
The above link is just one example. Look for "panel mount usb" and you'll find dozens of such solutions with *various cable length*. You can have a look at Aliexpress to begin with. I don't quite know what your application is, or the quantity you're going to make, but I personally wouldn't bother with wiring my own panel connectors when there are ready-made solutions out there.
Also unless you're going to mount this in a very tiny box, some extra length of cable is not going to be a big problem. You can pretty much do anything you like with the cable without harming communication a bit.
You could look for panel-mount USB connectors with a header connector on the other side rather than a cable. I think those exist. And then buy or build your own cable with a matching header connector. That's how external USB ports in PC cases are connected to motherboards. That works fine. And especially for USB full speed:
With that said, I wouldn't worry too much about USB specs here. The Pi Pico is just USB full speed. You'd really have to be very messy at wiring your own connectors to have it violate the USB specs.