> I hit the breakpoint in quit.  It looks like Fnconc uses FOR_EACH_TAIL,
 > which uses FOR_EACH_TAIL_INTENAL, which calls maybe_quit.

Not just around the corner but near.  Well done.

 > The question
 > in my mind now is whether block/unblock_input belongs in window_list or
 > in Fnconc.

In window_list and we then should bind inhibit_quit IIUC.  Fnconc must be
able to deal with circular lists.  I attach a patch.

martin