I've got so many questions regarding space flight in FG right now, but I guess each of them can be answered with "Yes, it's possible, but someone needs to implement it.
Well, one can always try...
I made some good progress today with figuring out how to code the RCS control logic. I've got the roll axis sorted out already, the others are a bit more tricky, pitch depends on CoG and yaw always generates a roll component which needs to be countered... And I did checks that the thrust agrees with what my book says.
I've also been cleaning up the HUD to display some stuff that's actually needed for the shuttle - like apoapsis and periapsis, total acceleration, current mode for the controls, ...
What's a bit annoying is that we can't initialize in space - so I always have to launch into a short ballistic hop to test the RCS setup (well, I did dry firing tests on the ground of course). What's more annoying is that the FG interface (controls/ and consumables/ ) seems to support way fewer engines and tanks than JSBSim does - but even for a simplified RCS, I need a grand total of 17 thrusters (the real one has 38...). Well, there are ways to work around this.