Check out http://bazel.io/docs/build-encyclopedia.html#maven_jar. In the root of your build, specify the jars you want from maven and then add them as dependencies in your BUILD files. The first time you run "bazel build", they'll be downloaded and cached from then on. It's somewhat limited in functionality at the moment, but should work for basic "download and depend on a jar".
* Suppose I develop using multiple repos and http_archive. I'd like to make changes both to a library and to a project that depends on it simultaneously, without committing the library patches to master github repo just yet. Is there a way to configure the http_archive, let's say by saying "bazel --mode=local", and have it customize the remote archive http to use a different url (say, my github's fork instead of the master github) for that build?
Scala support: yes, add using Skylark. Definitely let us know if you run into any rough edges, Skylark is a work-in-progress.
For multiple repos: there's no command line flag, but you could change the WORKSPACE file to use http://bazel.io/docs/build-encyclopedia.html#local_repositor.... Unfortunately, this may be of limited use to you. At the moment it's optimized/bugged to assumed that your local repos don't change, so it won't rebuild them (this is great for things like the JDK and gcc, but not so much for actual in-development repos). Feel free to file feature requests for any functionality you need, I'll be working on this a lot over the next couple months.
For multiple Github repos, use http://bazel.io/docs/build-encyclopedia.html#http_archive or http://bazel.io/docs/build-encyclopedia.html#new_http_archiv... (depending on if it's a Bazel repository or not). Let us know if you have any questions or issues!