I think the RU's experience, since we know contact with the MUs is possible, could help them deal with it- if it comes up.
The Caeliar were the only way to permanently deal with the Borg in the RU, so unless the RU Caeliar somehow absorbed the MU Borg as well, or this whole thing coincidentally happened on its own
in the other universe (with Mirror counterparts of the Caeliar, the Columbia
crew, etc.), I don't see how that is relevant.
In fact I'd wager that the latter explanation would be pretty much impossible, since much of Destiny
depended on characters from the 24th century - who either have not yet been shown to exist *at all* in the MU, or who are so different there that the chances of Destiny
happening in the MU are so absurdly remote as to not even be worth mentioning.