I have tried it once. There is the possibility to sit in multiplayer using a plugin and in Creative using an invisible horse. Your solution is, as you pointed out, for vanilla, so also single player. Forgive me, visually it reminds me a bit of a toilet. Other than that, yeah, it works. Except if you get to W briefly, then the lore drives logically. 

In summary: Usefulness good; optics rather less so

I like your idea in principle, but as @HipsterBanane mentioned, the optk of the chair is not that nice in my opinion. I'm not sure how, but isn't there a way to create working chairs in Minecraft that look really good (apart from the invisible horses)?

Doesn't this idea already exist? In (estimated) about 4-5 variants?