I'm carrying from where 'someone' left off trying to use standard Shimano trigger shifters with a Nexus (7) roller hub via a TravelAgent. I've been using an icky Nexus7 combined gripshifter and brake lever and although it sort-of works, it isn't very precise in the feel and I'm always expecting it to break.
I want to use a pair of Shimano STX-RC trigger shifters (I use a double 48/42 chainset up front), and asked the question of the excellent Harris Cyclery if there was any further information over and above what I had gleaned. They replied below...
....Yes, you can put a shifter together with a ratio-changer.
You need 5.5mm of cable pull for each gear change on the Shimano 7s and 8s.
If you wanted to use a Shimano 7 speed derailer shifter, it pulls 2.9mm of cable for each shift. So you need to find a ratio changer that converts 2.9mm in to 5.5mm output.
That's a 53% change.
You'll need to do some research to find the right ratio adaptor. Sorry, I don't have that info here.
.... So yes, it's possible. I'll be buying a Travel Agent soon and will double-check that the pull amendment is circa +50%,then proceed with the build. If anyone has done any further work on this, I would apprciate their input.