tox should grow some support for converting/running from .travis.yml files. Via twitter @mgedmin @jezdez @dstufft all gave useful hints. Projects to look into for inspiration or integration:
travis-solo https://github.com/jstasiak/travis-solo (via @jstasiak )
I think that generating a tox.ini on the fly could work well. It might make additions of features to tox neccessary. For example, it would be nice to be able to specify platform-dependent extra commands for installing debian packages etc.