The lfs migrate patch http://review.whamcloud.com/6662 will only allow migrating from a 1-stripe directory to another 1-stripe directory. That is unfortunate given that DNE2 is implementing striped directories. At a minimum this limitation needs to be documented for lfs migrate in the 2.6.0 release.
I think that implementing migration from one striped directory to another striped directory (restripe) is not very much more complex. Essentially, walk the source directory(ies), hash the name, migrate it to the correct target directory stripe. For migration, the "right" directory will never be the source directory, but for restriping a single directory to multiple, some of the entries will already be in the right place (stripe 0) and others would need to move to stripe N based on the hash. If changing from M to N stripes or changing the hash function, 1/M entries would already be on the right MDT and wouldn't need to be moved.