Thank you, everyone, for all of your suggestions!

Frank & I are making headway by using a mixture of c/t and natural horsemanship.
We are offically one week bite-free!

It suddenly hit me that Frank was getting really bored, and that was
contributing to his biting. I needed to find his motivation so that he would
want to work with me.

We added obstacles to our groundwork exercises (bending poles, trot poles, a
platform to stand on, etc) and he now trots up to me and puts his head down for
the halter when I approach him in his paddock. He also prefers to get to "work"
as soon as we get to the arena! I used to let him roll & run around for a few
minutes before we started groundwork, and he doesn't want to do that any more.
He now stands next to me with his ears eagerly pricked forward, waiting to get
to "work". Oddly enough, I've discovered that he prefers praise to treat
rewards! Frank is WAY smarter than I have been giving him credit for. If he is
able to bear weight, I'm thinking about doing arena trail competitions with him.
I think he would really love the variety and would thrive at having a job that
is intellectually challenging. If he can't bear weight, we'll work on trick
training.

We still have a long way to go, but every day that we go bite-free, I gain
confidence. Now I feel that I have the tools to become the leader that Frank
needs and wants.

Thanks again, everyone! I'll keep you posted on our progress. :-)

-Nancy & Frank the Tank