Most current MTB bars are designed for open face stems so how sharp the bends are is not a concern to the makers or users.

You can look around for older riser bars with gentler curves which will fit through the quill stem or get a quill to threadless adapter which allows use of a current open face threadless stem on a threaded quill stem intended type fork. I have seen the adapters on both the Harris Cyclery and Velo Orange web sites.