I mean, there's a reason we're not commonly doing this: a setup where the relaying and the back-end component are from the same organisation make it fairly usel

I mean, there's a reason we're not commonly doing this: a setup where the relaying and the back-end component are from the same organisation make it fairly useless. You need to find an established partner that wants to work with you to have some claim to not be colluding. And even if so, the front-end can always deliver another piece of javascript that carries off a copy of the plaintext to a server of their own. This setup has some clear disadvantages over Tor. But to repeat what I said before: I shouldn't be too critical when all I'm really saying is "it might not help enough to be worth the effort". If there are no downsides either (well, the effort, but that's up to the dev and consequently the user who pays the dev indirectly) then it's better to do it.