I think this is a critical piece of software because it is used by a
lot of applications and programming language runtimes.

Unfortunately only the most primitive mode is supported (single
threaded application - without any incremental) tuning.

I was really surprised about this fact. I don't have the time and do
not know anything about the OpenBSD memory system internals to do it.
So i would like to see somebody volunteer on this task. There might
also some kernel patching required as incremental garbage collecting
does only work well with some more information about the dirty pages.
WindowsXP offers an excellent "GetWriteWatch" feature to do this,
there is AFAIK nothing like it in the BSD world.