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Well I can't see any reason for the bar end shifters not to work in friction mode.
As for indexing, have a look at Sheldon's cassette spacing crib sheet.
If Deore 7 speed is IG then you have a chance at fudging it. If it's HG then probably not.
A quick Google suggests your 100GS (1991/92?) rear mech is probably HG, as IG only appeared (briefly) from 1994/95 onwards.
So I'd say friction is your only option with that combination.
I've got a 9 speed Deore gearset with 9 Speed bat ends.
I also have a 7 speed low spec 90s Dawes MTB with a shagged pair of 7 speed shimano 100gs trigger shifters.
Trying to do this on a budget and thought I could get some of those shifter mounts and either run the bar ends in friction mode or see how the indexing works.
Any thoughts on whether this will work?
Cheers.