An update:

I now have the cats at home. Plimsole (the cat with the ulcer) has eye
lubricant/interferon drops, an antibacterial ointment and lysine
tablets. Adminstering the medication is difficult but I'm learning
fast.

He doesn't appear to be experiencing too much discomfort, based on how
often he washes his eye and minimal squinting. He is a totally
different cat now that he has a house to roam around in. My furniture
is taking a bashing: he does like to hang from it during his sillier
moments.

The vet he was seeing during his stay in the shelter was considering
removal of the eye if the ulcer hasn't responded to treatment in the
next 8/9 days. I don't want this!

I won't give up without a struggle, but ulimately I need to see that
he is comfortable.