One question I'd ask is whether the build machine runs builds
automatically. If it's automatic, I would think that all it needs
is the output files from the people authoring the help. Don't you
need a license only if you're actually going to be opening RH and
working on a help project? If this isn't going to happen on the
build machine (if all the build machine is doing is taking output
files and rolling them out to a server), I don't see why you would
need a license for it.
But if someone is manually running the build machine and
needs to be opening/working on/generating help projects, then yes,
you'd need another license.
Just my two cents,
Ben