-I've used the 220#@30" wytch elm bow as a foot bow, and it's a lot of fun. Actually, you won't use your arms much. You hold the string with both hands while your legs are bent, and then just straighten out your legs. Remember to tie the bow to your feet.
It was not at all difficult, and it wasn't any hard to "pull" (push?) the bow at all. I could have shot a much heavier bow this way. What was hard, was aiming and not overdrawing the bow, as it's hard to see where the arrow stops while lying on the ground like that.
I guess if you make a bow from osage, yew, wytch elm etc., you can easily make a 250- 300#'er if you just make it long and wide enough. I guess if I were going for 300#, I would have made a w/d 7cm/5 cm at handle, 230cm long ELB style bow from wytch elm. (Hmm, maybe I will..?) I'd also give it a thorough heat treating on the belly.
-I also guess osage would be perfect for such a project