Using CONTROL= with "%u" will quite quickly run into the problem that the directory will fill up, and hit the maximum (around 32k) or so. We have 170k users so far, and more are being imported. You could cull some from crontab or similar, but is generally a poor way to solve a problem.
Worse, it also moved all the "subscriptions" files, so customers no longer saw their folders, and different result in each cluster. (it'd would have to move to a NFS volume).
Seems the only real solution is for us to patch Dovecot, or hope for future features.
Lund
-- Jorgen Lundman | <lundman@lundman.net> Unix Administrator | +81 (0)3 -5456-2687 ext 1017 (work) Shibuya-ku, Tokyo | +81 (0)90-5578-8500 (cell) Japan | +81 (0)3 -3375-1767 (home)