Back a couple of years ago, when Campy was selling 10 speed groups and Shimano was still selling 9 spd, I bought an Ultegra cassette that had been converted to Campy 10, to meet just that situation. (have a Chorus 10 group)I've had that cassette on two different wheelsets, and it has performed just fine. Mine was made by Wheels Inc, ordered it from aebike.com, though I think Branford Bike also carries the converted cassettes.
I have heard that Campy 10 and Shimano 10 are very close in spacing and can be interchanged, but have never actually tried a Shimano 10 cassette on my bike.