This thread isn't about that, but do you have an exact quote or link to something FAS said about it being impossible?
For
@Bruno Ferreiro : I think it's been mentioned that external latching switches currently can't work properly as stand-in switches for FCx tap functions, even if you configure the Axe-FX to know it's a latching switch. That would prevent using CS Per Scene states with the switch as a stand-in for a Control Switch. (Maybe
@unix-guy can verify this to save you the trouble of doing the next suggestion if it turns out this actually works.)
What could work is to assign the switch to bypass functions of several blocks in the Setup: Midi/Remote: Bypass menu, and "Eff. Bypass Mode" to "Toggle" at the Midi/Remote: General page. Then use the block # from the first step when you want the switch to control it, and a different instance/number of the block when you don't. (The state of block #s you did assign the switch to could also be overridden with a CS as bypass modifier, using the CS Per Scene menu to set the state.)
Maybe important note: I just tried this on the Axe-FX III with a switch at the Pedal 2 jack, and the "Toggle" bypass style doesn't actually work properly. It can only bypass a block, then subsequent presses do nothing. I don't know if the same would happen with external switches and an FMx or FCx.