Is it possible to instruct fvwm to let pass modifiers to specific application?
Well, stupid question. Of course, but how?
This is for example to handle another windowmanager instance in a Xnest window.
No – not in the case with Xnest. AFAIK, this is a documented limitation.
– Thomas Adam
Ouch. What a pity 
Ok, nice to know, so I search the Documentation of that missing feature
instead of the function itself 