Custom Validator still seems like the best option: http://web2py.com/book/default/chapter/07#Custom-Validators
Although I think unique_together is implemented via db constraints. That seems like a very remote scenario so probably not outlandish that it takes a little code. Postgres, at least, automatically creates the index for primary keys. Otherwise, Web2py does not create any indexes. Validators, by the way, are enforced at the form level and have no direct effect on the db.
