Google Code projects can have multiple hg repositories (http://code.google.com/p/support/issues/detail?id=2563), but there appears to be no way of configuring Hudson to use the repository browser for a repository apart from the default one.

The base browsing URL for a default repository is
http://code.google.com/p/PROJECTNAME/source/
but if an extra repository called REPO is added, the base URL should be
http://code.google.com/p/PROJECTNAME/source/?repo=REPO

It looks as if Google Code just adds the parameter repo=REPO to all the browsing pages, eg
http://code.google.com/p/PROJECTNAME/source/detail?r=123456789&repo=REPO

There's an example of such a repository here: http://code.google.com/p/living-stories/source/browse/?repo=wordpress

Note that repo=default does happen to work for the default repository, but is of course not required.