Recently fitted XO shifters and while I had the chain off I gave the existing Shimano LX front der a clean and checked the adjustment. I noticed that minor adjustments to the low screw cause definite and noticeable movements of the cage but the high screw seems totally inert. I could screw it all the way in and one heck of a way out, but the cage just does not seem to move. Yet, the chain holds fine and there's no slip.

Does this sound like the high screw is a dead 'un? Maybe there sumtink broke?

If the threads aren't stripped,and the screw moves when you trun it,it ain't 'dead'. In the low position the spring tension holds the pivot against the screw so that movement is noticed..Screw the hi screw all the way in.Use hand pressure to hold the deralier against the screw.Now while holding pressure,back the screw out.The deralier ought to follow it.

you can also see this if you shift into the big chainring, and adjust the screw, but be careful not to damage the screw when it is against the pivot.