Simon,
Can you describe what situations you find yourself shooting the 500 @ 1/60 and slower? Is it landscapes, astrophotography? I'm looking to expand my horizons, so to speak, with this lens, and am open to new ideas. What I've been doing so far, shooting wildlife, renders slow shutter speeds and mirror lockup moot.
I'm curious as to what way the Aria exhibits excess vibration due to mirror slap. In the short time I've had mine I've found it easy to handhold at very low speeds (although admittedly I keep the speeds high if attempting to handhold the 500). It seems a well-d&ed camera to me.
As a thought, if you only need mirror lockup for the 500, you can also get it with an RTS I or II for a lot less money. I'd also make completely certain that any blurring from motion isn't coming from the tripod or head. In my experience it takes a really massive tripod and ballhead to do an adequate job with longer lenses, e.g., a studio Gitzo and a Linhof or Arca-Swiss ballhead.
In the meantime, I'm playing with faster film and a monopod.
--Rick