So I finally solved the problem of inverting Gerber layers in Altium, it required a trick to maintain the origin, but here is a guide for anyone attempting the same in the future:
This of course messes up footprints etc. but for a front panel where you only have text on one side and no electrical functions this can be interesting to test. When I have time I will order a test board to see how it looks.
Run the "Film Wizard":
Click the button for Neg./Mirror layer, mark the Layer and click OK:
You will then have two layers, of which one that is inverted, much bigger and with the wrong origin, but otherwise correctly drawn:
To fix the incorrect origin we can use the "Align Selective" function under "Edit":
Then we click one of the objects in each of the layers (the order is improtant) and the inverted "film layer" will jump to the same location as the orginal:
We can then save the "film layer" as a new Gerber, and if we open it in Altium it looks correct and it also uploads to PCB fab correctly (just tested the online preview at OSH park which looks good)