I have used the sapwood before as a back..But never just under the bark on an actual stave(usually to thick and weak)..Always chased it down to the trasition ring,or the "Creamcicle" ring..I did just use the sapwood directly under the bark for the back on a lower poundage "D" bow made from a large Osage sapling. Some guys on here have used the sapwwod for a back on Osage with no problems..In my expierence the sapwood on Osage is , for lack of a better term,soft and chalky...As long as the sapwood is sound and you use the right design,draw,and weight..etc. Ya might be ok..But why take the chance on a good piece of hedge...Just my 2 cents tho..