I'm in favor of a separate repository. We can simply build all of the components we need and enable/disable them depending on the discovery of available runtimes (rtgov, dtgov, s-ramp). I think that will be much easier than trying to create tooling in all the sub-projects and somehow integrating them.
It also means that the GWT build cycle for the tooling doesn't slow down the builds of the runtimes. We have enough of that already.