Remember that Mahout is an open source project and, as such, doesn't really have a roadmap. What does exist is a set of desires that contributors have. As the contributors feel a need for something, it happens.
This means that you guys can influence the future of Mahout quite heavily.
To your point, however, it is true that the clustering code is rather inflexible about input. So is the Naive Bayes classifier family. The recommend framework is much more flexible (Sean recently added a Cassandra interface with very little work, for instance). The SGD classifier family is all about in-memory API's which makes it pretty easy to interface with.
The primary limitation right now on how the clustering and Naive Bayes systems accept data is that there is very little consensus on how that should work. Your input would be very helpful here.
Try emailing firstname.lastname@example.org
and start a discussion around what you need.