Has the cegui team considered setting up some kind of continuous integration? (Or is there some kind of continuous integration system that I'm not aware of?)
One nice way that you could do this is, set up some kind of automatic github sync that automatically pushes any bitbucket updates to a github mirror, then, activate travis-ci on the github mirror. You can make it build and run tests with every commit to a collection of (whitelisted or blacklisted) branches, and post updates in irc or to an email list. (Also you can make it build pull requests, although probably it is hard to mirror pull requests so likely you would disable that I guess.)
In truth I would like to see some kind of auto github mirror anyways because I have way more experience with git and I'm pretty slow with mercurial.... I tried to figure out how I can make a github repo that automatically tracks the official cegui repo, but it seems like the easiest ways are for someone with administrative access to cegui to attach a service to the repo which pushes updates when a commit happens.
Having an well-maintained github mirror makes it easier for people who are more experienced with git, because they can just clone the mirror, do their work using git, git rebase, etc., then use
git format-patch to create .patch file(s), then clone upstream cegui on bitbucket, clone locally using hg, make a branch or whatever, and use
hg import to import the patch files so they can make a pull request in bitbucket.
Ability to use travis-ci is probably the best non-selfish reason to consider this.