The offline defrag won't actually "touch" the original database file, rather it creates a new one and re-constructs the database there. Technically then, you should be able to cancel it and remount the database.
(quote from MS site)
Defragments the database offline but leaves the new, defragmented database in the temporary location with or without overwriting the original database. This mode reduces the gross size on the disk of the database (.edb) by discarding most empty pages and by rebuilding indexes.