I believe you can still transfer TPUs. There's a fee and a form. That will sometimes be done when a person sells an underlying week and agrees as part of the terms of the sale to transfer the TPUs generated from the deposit of that week to the new purchaser.
However, to sell the TPUs is considered by RCI to be a commercial use of your timeshare. Just as you cannot make an exchange with TPUs and then rent it out, you can't sell the underlying TPUs. If RCI gets wind of it, they could confiscate the TPUs and close your account. Even if you're not selling at a profit, just recouping your fees, it could get you into trouble.
Now, having said all that, there are lots and lots of people who rent exchanges. Take a look at Ebay and you'll find a whole bunch.
There are also lots of folks who might occasionally "give" an exchange to a "friend" and the "friend" may return a "gift" of cash.
My personal take, and this is strictly my personal take. If one transfer form came through to someone for all of your TPUs, I would think that RCI wouldn't get too excited. If you were parcelling them out in a number of transfers, it might raise a red flag.
FWIW, I feel your pain. I have A LOT of TPUs that I will probably not be able to use. It's frustrating that the rules don't allow at least a reimbursement of maintenance fees with no profit, but it's their sandbox, their rules.
I hope I've been helpful and not clouded it even further.