if i remember,the rear mech is not the issue(unless its one of those weird sram things)
the deal breaker is the rear hub/cassete
campag and shimano have difering spacers betwen the cogs-obviously a campag shifter is designed to move enough cable for the spacing of the campag cogs,using a shimano hub/cassette will probably index ok across the middle but will go progressivley further out of sync as you go to the extreme ends of the cassete
it is possible to buy campag casette spacers seperatly from mercian etc and maybe use these to alter the spacing on a shimano hub i also think highpath engineering do adapters of various sorts to allow various hybrid mixes to happen
if you go really really retro and go friction everthing works then