Here is an attempt:

  .de Q
  .nf
  .na
  .pso awk 'BEGIN{bs=sprintf("%c",92); pre=bs"&"} {out=pre; for(i=1;i<=length($0);i++){c=substr($0,i,1); if(c==bs) out=out bs bs; else out=out c} print out}' "\n[.F]"
  .ex
  ..
  .Q

Invoke with:

  nroff -U -Tascii quine.roff | sed -Ez '$ s/\n+$//'

Possibly relies too much on awk + sed. So maybe not A+, but better than nothing.