Question

Yeah, but that would probably be messy, it would require changing the database schema and hacking MW

Maybe add a "mod block" thing to the vandal brake?

Suggestion - why not open an ER in the Mediawiki bug tracker? I think what you are talking about here is an interesting use case, which may be of interest beyond just RW. Even if the Mediawiki developers don't do anything, if you make a change you could always work on getting it upstream (then you won't have to maintain it anymore...)

Ok, here's an idea: If the blocked user's user page is protected at the mod level, their block can only be changed by a mod. It's a hack, but it's pretty easy to implement.

Dammit, there's no hook for unblocking, so I can prevent changing blocks, but not unblocking. I've got to go now, I'll get back to this later.

Well shit, I was hoping unblock was treated like a change block....

Editing MW so that unblock = change block to 0 would solve this, I guess, but may be way too much hassle.

We don't want to edit MW, maintaining a patch is too much work.

Do it properly and you can upstream your patch.

Been there, done that, getting a simple patch for an actual bug upstreamed takes forever, can't be bothered.