Robbie, You were on the right track to start with. All derailleurs pull are analog. An indexed derailleur is just matched for cable pull to the distance a derailleur has to move between cogs on the freewheel or cassette as you said. I have used 6 speed indexed Shimano thumb shifters with Suntour freewheels with success. The tolerance for a 6 or 7 speed are wider than those for a 10 speed obviously. Try it out! All you have to lose is a cable. There seems to be a big difference in Campagnolo cable pull from Shimano in the new stuff.