The Lowdown:
I guess we'll never REALLY know until the writers create the situation and define the parameters. We can simplify the question down to "Can the Borg assimilate the Founders?", the answer to which we don't quite know either; the Borg nanoprobes seem to need a creature with a cellular structure in order to assimilate it. While earlier episodes of DS9 seem to indicate that this isn't the case, the episode in which Odo finds a baby Changeling ("The Begotten," I think) does make mention of some sort of cellular composition for Changelings. So I would hazard a guess that the Borg would be able to assimilate a Changeling in that it can introduce nanoprobes into its cells; whether the Borg can subvert the will of a Founder, I don't know.