What about those hard-rubber dog bones from the pet store? They are non-toxic, can take a bite w/o breaking teeth and the knuckle on either side lets you tie it on. The also come in various sizes and lengths (but I've never seen black.)

If you have a medical/dental supply store - they sell dental wedges which can be used to hold the teeth open. While not a gag - it does remove the sub's ability to speak.

Be careful with ball-gags/eggs. My wife had serious TMJ issues a few years back and went through braces and some teeth shaving to solve it.