That the DO could theoretically transmigrate the soul of a balefired person, if the amount of balefire was very small. We don't know exactly, though, if Moiraine's balefire was such a "very small amount".

Besides that, this information only came from a Tor Q&A outside the books. For a "normal" reader, who just knows the books, it's a rule that the DO can't transmigrate a person, who was balefired. It would be very sloppy writing, if RJ suddenly reveals in one of the last books that a balefired person can be transmigrated, without indicating at all in the books that this is possible (but actually stating that this isn't possible). And if it's supposed to besome twist, RJ certainly wouldn't already give it away in a Q&A.

Therefore I believe the "very small amount of balefire"-exception to the transmigration-rule is just an academical information by RJ for interested fans; it isn't supposed to hint at a transmigration of Be'lal.