Are we really considering this?
Fix a batten to a wall...just do it!
Fix cable trunking, mini trunking, whatever, to a wall over an exit...just do it with fire-safe clips/galvy band
Use some wiring to fill the gap if the screws don't bite?
FFS! The world is full of fastening solutions!
Rawlplugs/walldogs/metal screw fixtures/concrete plugless self-drilling screws/self-tapping screws/hammer-in fixings/expanding bolts/resin bolts and studs...if my builder used bits of offcut cable I would be seriously ----ed-off.
but then...I HATE builders! So pay no attention to me...