i could totally picture ulfric joining forces with the redguards. i mean, certainly he's smart enough to know that his forces alone will not be enough to take the entirety of the aldmeri dominion, which fought the whole empire and nearly matched it. they certainly have hatred for the empire in common, and a general distrust of elves.
from an imperial viewpoint, it would probably just be trying to regain hammerfell as an imperial ally, but i'm not too sure if anything could really be done to regain the redguard's allegiance.i mean, the empire tried to give a whole portion of hammerfell to the dominion. just imagine how pissed the nords would be if they tried to do that with skyrim.