Campy 8sp shifters and derailleurs should shift okay on a shimano 7sp cassette. Of course you'll have only 7 speeds. Generally campy cogs are thicker, but the overall spacing is the same so it should work. Note that if you have two wheels, one with Shimano 7sp and the other Campy 8sp, you'll have to fiddle with the derailleur limits each time you switch. You could also try a shimano 8sp cassette with 7sp spacers.

Apart from the Jtek adaptor and WM spacers, I think Miche makes hybrid cassettes for campy/shimano.

Finally, 8sp campy pops ups reasonably priced once in a while. There is some on my local CL.