@aral
I think it's much more effective to give the responsibility of encouraging people to use alt-text to whichever client they use. This is both easier and more respectful to the users.
If someone toots without alt text and everyone complains, they will often respond in a hostile manner or start disliking the fediverse.
Besides, I think many new people simply don't know what it is and why they should consider doing it. Including a simple hint in the client will solve this for everyone.