Yeah, I think an appearance is out of the question. https://www.theguardian.com/science/...onservatorship
(Given Nichols age and illness I don't think this is anything like the Britney thing despite the similar legal issue).
If any TOS cameo takes place Takei is probably the best choice, outside of Shatner he's pretty much the most active TOS member. Given how he's still doing a ton of voice roles, maybe have him voice an alien?
I wouldn't rule out a new Sulu either at some point given that he was early in Kirk's crew along with Spock and Scotty, although not at the helm yet.