I haven't personally engaged in this...give me time! But it sounds like something that a decent set of nipple clamps would be suited for. I purchased the type that have a screw for adjusting the tension. The adjusting screw prevents the jaws of the clamp from closing completely. That way they maintain a firm grip without inflicting overly distracting pain or damage. I'm sure they could be easily modified for your purpose.

Of course there is always the option of piercing the labia... but now I'm letting my imagination get the best of me!