I think abusing a write-off electron microscope to side step the need for masks is also an interesting idea, however, I believe acquiring wafers of sufficient quality and depositing layers to be etched could be the bigger challenge here.
I meant "drawing" on a photoresist layer with a SEM and then wet-etching it. Also all silicon in a photodiode is doped, so the etched parts would be of little use, I believe.
Definitely hard for a home fab but how about a community fab? Not necessarily a geographic community.