This is actually an upstream feature request in the code we added to buildbot, but we are the ones who contributed the pertinent code, and who care.
Right now, every time we need a new slave image, we actually need to make three new images: devel, db-devel, and bzr. Other builds would mean other slave images, such as bug 383348.
This is time consuming and wasteful. This is particularly annoying because the difference between these images is simply a name and password in the buildbot.tac file.
Fixing this could be done by making the master able to initiate conversations with the slave, rather than the other way around. I am told that the underlying Twisted protocol is symmetric, so this should not be too difficult.
Alternatively, I believe there are AWS/EC2 tools for providing data to an image as it instantiates. The master could provide the necessary information, and the image could have the necessary smarts to put it in the right place.
I think it would be reasonably possible to use "user supplied instance data" http://