I believe Hackage already supports urls like that if you leave the version out: http://hackage.haskell.org/package/base/docs/Control-Monad.html

This automatically redirects you to the latest version.

The problem, then, is visibility both to users and to Google. Since it's an actual redirect, your URL bar will show the specific version when you follow the link. The version number will also show up if you copy the URL from one of the links on the contents page even if you got there without specifying a version. (By following http://hackage.haskell.org/package/base for example.)

I'm not sure if this is an intentional design or just a quirk of the implementation.

One thing to keep in mind: how would you handle "latest" URLs for modules that have since been removed from the package? Presumably, we'd still want something showing up in Google results or if people follow an old link, after all.