So, here's what I've decided:  I'm not going to do the job of a buildbot:
running the normal darcs unit tests on different platforms.  However, I will set
up and maintain a buildslave which will do that for me.  I can get my company to
let us use a Windows, a Cygwin, and a few different Linux systems, and I'll use
my own Solaris system.  Then darcs can have a set of automated buildslaves
similar to this set: http://allmydata.org/buildbot/waterfall?show_events=false

I will also help set up and configure the buildmaster.  The buildmaster issues
instructions to buildslaves, saying, basically "Do a darcs checkout of the
latest version, do a make, then do a make test, and report back the results.".

The buildmaster can be triggered to issue these commands any time a new patch is
pushed into the darcs repository, by editing the _darcs/prefs/defaults and
entering some sort of trigger script in there.