Undoing transactions in a broken Plone/Zope

Today there was a situation where someone made a permission change that made Plone inaccessible. The ZMI was also inaccessible (by design (not mine)). We could get access to the filesystem, however. So after a couple attempts at undos through in debug....There were issues with permissions and RESPONSE objects and all the other Zope machinery. So I tried mounting a ZODB directly and undoing transactions. The following worked great for me:

filename = "/Users/reedobrien/instances/zeo/msrd/var/Data.fs"
import ZODB.FileStorage
import ZODB.DB
storage = ZODB.FileStorage.FileStorage(filename)
DB = ZODB.DB(storage)
txs = DB.undoLog()[::-1]
DB.undo(id=txs[0]['id'])
import transaction; transaction.commit()

Maybe the wrong way and YMMV, but it stayed in one piece for me.