I have now also successfully removed the FAG bracket from one of the frames and I'm now rather confident it can be done like this:
- Remove the flanges (just try to unscrew them, they'll rip right off)
- With a chisel or a flat-tipped and expendable screwdriver remove as much of the material from the inside as you can, but try to pry it away at the cartridge and steer clear of the threads (I actually drilled holes into the plastic with an electric drill, this also helps remove a lot of the plastic)
- With a soldering iron use the thinnest tip you have to melt and pry away as much of the plastic as possible
- With a hammer, punch the spindle towards the left and see if something moves, if not, remove more material
- If the bracket moves out to the left but then gets stuck, punch it out the right.
This finally removed by patient, but the threads took some damage. I think with a thread chaser they might be restored sufficiently to take a new bracket, but I'll need something to test it.