One of the posters here mentions using venison hearts w/o problems. I certainly can't see why there would be any issues w/ them, or elk, buffalo, pronghorn hearts. Prep would be about the same as for beefheart, I suspect.
Heart is the lowest fat of any cut of meat, and also one of the cheapest, but I wouldn't have any qualms about blending in any other really lean cuts into a batch of beefheart mix...
That's just me, relatively inexperienced.... maybe others will chime in...