Z80 united

Part of the problem is that graphic calculators ARE general purpose computers. They're fully programmable, run a bastardized pseudo-BASIC and all seem to have active devscenes...

Also, would terminal-based CP/M boxen be in the running or would having graphics and sound abilities be part of the qualification?

Let's not find problems where they typically aren't .. Unless someone can give me another brand, let's stick to msx/spec/cpc.

Colecovision?

Well, I'd like to say that Coleco, SMS, GG or even Colour/classic GB with its Z80-alike are all of them systems with an active scene. I don't have details about Amstrad PCW development, but I'd say that it is also a Z80 based computer...

And to remain consistent, Turbo-R users should be banned from coming in, at least in R800 mode

I guess it has something to do with gameconsoles as well... you need writeable carts for the gameboy, sega mastersystem, genesis
and colecovision in order to produce anything at all, unless you do it on an emulator basis. All of the devices don't have diskdrives,
but there is at least a large gameboy composer scene, consisting of over a hundred people. As for the sega, I wouldn't know.

I guess there is hardly a scene for the sg1000, since they are pretty rare. Never see them pop up on ebay either...
same for the jupiter ace (nice looking thing)... maybe the sam coupé (another z80 thingy) has some following,
I believe chances to stumble upon a small community of those users is more probable then the sg1000,
which I think was mainly a jap thing