I cannot imagine why the set up won't work, I'm interested in what the mechanic says.

I used 9 speed (new-ish, about 5 years old) Dura Ace bar end shifters with a 9 speed 11-34 MTB cassette, 9 speed Deore XT (MTB) rear derailleur and a MTB front crank on my hybrid turned drop bar commuter. My guess is that my crank was either the same as yours or maybe a 44 big ring. Either way, my chain wrap capacity need was probably the same or more than yours.

As far as I know, the compatability issues are:

9 speed bar end shifters proper throw w/ 9 speed MTB RD - CHECK
9 speed bar end shifters indexing w/ 9 speed MTB cassette - CHECK
Max cog capacity of MTB RD w/ a 34 tooth large cog - CHECK
RD chain wrap capacity with a 20 tooth front difference and a 21 tooth rear difference (41 teeth total) - CHECK
(should be anyway because both of those are well within "normal" for a MTB set up.... unless yours is some sort of shorter cage version that doesn't have adequate chain wrap?

all these things should work which leaves possible rear spread issues, (maybe the spacing's too wide?), mounting of the RD or something I'm just not thinking of. But I think it should be plug and play.