Unfortunately, I don’t think you can get what you’re looking for @andy. Note that while this was built in to the TAP protocol, part of that protocol is that it is allowed to send you duplicate items. In the current (and only) implementation, if you send any timestamp in the past, it will send you all of the data.
The original plan was to later extend it.
Plans change however, and that was taken a different direction in Database Change Protocol (DCP). As you probably already know, DCP gives you the ability to pick up from a given checkpoint and that’s the best way to handle this.