To be honest, I didn’t expect the thing to last that long though it was worth it. As the topic suggests, the beast is working now.
While the actual build script was done in one day, getting all involved things to play nice and friendly was really a hard job. Involved were cachings, optimizations for different VCSs and many minor corner cases.
Facing facts I’m abusing a system which was designed to fetch source from some repository then run tests on it and if multiple requests come in, just work on latest one. This is partially what I want and the other part is the opposite. So I had to dig deep into BuildBot to solve all outstanding issues.
Job management was more like coding top down (no really hard task after all) though needed tweaks along the road as well.
But as I said it was worth it. The thing has no dark corners anymore for me 
And: there are pre-stable tests now for the incoming requests 
Note for interested people:
Repo of it has moved to dev.gentoo.org/~jokey/tinderbox-bzr
URL of the BuildBot Status Page: http://buildbot.gentooext.net/
I need to get with you about this sometime – sounds very cool and I wanna know more!
what happened to this? how was the coupling of ebuild to buildbot done?