He's never been my Sirius of choice, but I think that's mostly because he's too old. Then again, so is Alan. Hm.

and would match up quite well against Rickman's Snape...

This is so very true. They would convey exactly the chemistry I appreciate about Severus and Sirius.

But - I'm afraid I wouldn't survive Alan Rickman and Jeremy Irons portraying my favourite slash pairing.

Voldemort is indeed difficult. you need someone able to project presence and malevolence without turning absurd - that's why Jeremy sprang to mind. Sirius is already taken, after all.