First, let's get the specs on the standard items:

Campy 9spd
overall width = 38.2mm
centre-to-centre spacing = 4.55mm between centre of cogs
cog thickness = 1.75mm
spacer thickness= 2.8mm

Shimano 9-spd
cog thickness = 1.78mm

Using the Shimano 9-spd cogs with Campy 9-spd spacers will give you c-t-c spacing of: 1.78 + 2.8mm = 4.58mm. That 0.03mm difference in c-t-c spacing won't be an issue as it's close enough. You'll need to shim the Shimano 4.34mm top-cog with integral spacer with an additional 0.2mm spacer to match the Campy spacing. Then you'll have a Shimano cassette with Campy spacing with minor variation.

To reduce any negative effects of compounding the differences as you sweep across the gears, adjust cable-tension so it shifts perfectly in the middle 3-cogs. Then you'd be compounding the variations only 4x in each direction.