I've got an old Deore RD with a Shimano 7spd freewheel and 8 speed Shimano bar cons on my Voyager and was just fiddling with it yesterday. For the most part it shifts OK but I get a little chain chatter on the last two (smaller cogs) on the freewheel when I'm on the big ring in front. I can't seem to adjust it out totally but I guess it is tolerable. Mid range gears are fine. The 8th gear (relaxed position on the lever) is the "non-gear".

Did Shimano ever make a set of 7 speed index barcons? I've given some thought of swapping my Durace 8 speed levers for a set of 7 speed Accushift if there are no Shimano 7 speed levers available. I'm using Accushift with a Suntour Derailleur and Shimano HG cassette on another bike and it is smooth as silk.