Is there a risk to the colony from a build up of dead bees on the floor blocking the exit in winter?
Not really - on an underfloor entrance if they fall over the entrance gap, a great thing called gravity means they will fall through onto the floor of the 'lobby' below. They don't stay there long as the wind will clear them.
It also means less work for the undertaker bees as all they have to do is carry the dead to the area of the frame over the entrance slot and drop them out.
I had a very strong colony last year that seemed to have suffered an early autun mass dieout of the older bees the pile below the entrance slot must have been nearly three inches high and spanned the whole width, there was a carpet of dead bees on the OMF inside as well.
Bees were still flying unhindered though and they came through winter champing at the bit to be one of the strongest colonies this season