[Rumil]

Somewhere in HoME in an early draft of LoTR Theoden receives volunteers from amongst the Dunlendings if I remember. So possibly!

Whether Tolkien decided against this idea or simply didn't think it important enough to include I don't know. I'd imagine that they would not make up a major force, as although they had some cavalry there don't seem to be many, so could not contribute that much to the ride to Gondor.

Sure they could have helped garrison Rohan, but the Rohirrim would have to be very sure they would not re-revolt when their backs were turned! So probably safer to send them home.

After the war, maybe Aragorn would be best to bring the Dunlendings into the fold, as he is not a traditional enemy and is also not blonde!