I think it would just get DROSed as a rifle receiver in that case. My understanding is that unlike the paper DROS, that the computer form will not allow you to pick pistol frame/receiver for a lower unless the transfer qualifies for a roster exemption. On paper, it used to be possible.
I'm not sure how the single shot exemption thing works anymore. I do think that the DOJ has gone beyond its rightful authority when it comes to roster exemptions or DROSing frames or receivers as pistols, but no one can really do anything about that.
In any case, a stripped lower or a complete one cannot meet the definition of an AW unless it is on the make/model ban list. No reason it should not be able to be DROSed, and if an FFL refuses to do it, I'd say find another FFL and stop patronizing his business at all.