I'm really happy to hear that you're at least considering it.
>We may have to restrict posts on the onion to be only text or image if we do implement a hidden service gateway.
A tip from someone exclusively using the 8ch .onion on the very few and obscure boards worth browsing, simply disable images. On 8ch this unfortunately also entails that thread creation is also disabled for the .onion users, due to thread creation being impossible w/o pictures. This I think was an unintended consequence, so I'm pointing it out here in hope that it won't be repeated.
The stereotypical threat of .onion access is CP images and perhaps CP hyperlinks. The first would be averted with the above restrictions.The latter, if some sort of defense isn't already in place (which would be useful) could be countered with whitelists, scripts. I think 420chan already does this IIRC.
Anyway, glad to see that Lainchan's going well. Thought it would crumble and burn forever there for a while.