Well... considering the pinhole is assumedly just a tiny hole in a body cap, and has no optics, I don't see why you could not put it on a FF DSLR body - the same body cap fits on all DSLR cameras.
Alternatively, could put it on the EF- RF Canon adapter and get any of the Full-frame R-series bodies. So that would be the R, RP, R3, R5 and R6 at this stage. There are other APS-C R-series cameras out there now: the R7 and R10, but they would have the same sensor as your 20D, so no real benefit there.
You might want to see if you can try this solution out first if you want to buy one.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
