/dotlrn is not a subsite, but an instance of the dotlrn package. I think it should be possible to create a /dotfolio subsite, and include all the user portfolio subsites under that. If it doesn't work, it is a bug.
The only place that would present a problem is if there are hard coded paths in dotfolio. dotlrn suffers from this problem that is must be mounted only directly under /dotlrn.
