Lint now automatically looks for extra lint jar files supplied with libraries. For example, the appcompat library (the AAR file that is) can contain a lint.jar with custom rules that perform additional checks for that library. (It doesn't yet, but I used that as a local test when adding the support in lint for this). Then, any app that depends on this library will automatically pull in these extra rules and apply those to the project. This should work in Studio (with gradle projects) today. It should also work with the gradle lint target, but the command line gradle integration is still incomplete.
Right now you have to manually assemble your AAR to contain the custom lint rule jar, but the plan is for it to get automatically built if you have a custom lint rule directory.