I've seen pictures of LHTs with flat bars... cerainly do-able. I'm still in the dream-of-owning phase w/ the LHT myself, though, and mine will have either drop bars or (see next paragraph)...

If I were to suggest another possible approach to the handlebar situation for your wife, I'd say look into butterfly trekking bars like these. I have had them on my bike (a hybrid that sees every-day commuting duty) since June and LOVE THEM. They're a great, under-considered option, and would work GREAT on an LHT. Nashbar and Wllbike have some, too. Just a thought. Good luck with the build!