As stated in our email correspondence, you can use a combo of DT Register and DT Donate to accomplish this. Using Pay Later in DT Register, then customizing the pay later thank you message including a link to DT Donate, users can then go there and setup recurring payments with PayPal or Authorize.net.
Eventually, we'll have this type of option built directly into DT Register, but for now, this would be the way to go.
I know this thread is very old, but, a bunch of us out here are still waiting patiently (and sometimes not so patiently) for this feature to be included directly in DTRegister.