Could Aikido be taught completely visually?
Yes! I haven't worked with a deaf person to my absolute remembrance (something is tickling my brain) but I've definitely worked with a blind person and with the exception of very minor adjustments he fit in fine. I would think that would be much harder than being deaf.
If you think about it, Aikido is taught almost exclusively by visual and feeling of movement. I think a very small part of the transmission happens verbally. So personally I can't see any reason the deaf can't participate (as I think on it, I'm sure I've worked with a deaf person before).
The Deaf are a fairly large community that are excluded from Aikido because of the need for expensive interpreters (up to $50/hr.) for the traditionally verbose classes.
I have to disagree with this. I doubt very much that any school I've spent time at would exclude someone based on deafness. They might have to modify things a bit and rethink how things are done but it could and would be done in my opinion.
My question to you is who is telling you that you need the interpreter and why because I question the need on a regular basis.
I'm sure others will have more to say.