I run Suntour retrofriction bar ends with my 7-speed touring setup, and it's surprisingly easy to shift into the right gear and stay there. OP may want to consider friction rather than indexed...

But, using the alternate cable routing on the rear Der with 8-speed levers sometimes works better than using the 8-speed directly, but I've used the 8-speed shifters on 7 speeds clusters successfully in the past. The Der is a little harder to keep adjusted, tho.