The member nidrinir on paleoplanet ( his website is
http://wwwnorwegeanwarbows,com// ) has made a good amount of elm warbows, of incredible weights. I remember a couple he posted with a bit of brown heartwood on the belly. I have also seen Druid on here post some very nice elm warbows, I think with a touch of heartwood here and there, if I remember correctly. I myself have no experience with elm, the only elm I ever cut checked too bad to make a bow,
. You might be able to contact one of those guys though and pick their brain about it?