I have a number of folders that I don't need to sync at every
offlineimap run. Therefore, I implemented the use of a sentinel file
to skip those except for twice a day in the folderfilter function.
Ssee py_folderfilter_seamus in
http://git.madduck.net/v/etc/offlineimap.git/blob/HEAD:/.offlineimap/helper.py

It would be nice if this functionality were available less hackishly
and at the level of the configuration file.