I consider it an important usage scenario so I will expend quite a bit of energy solving it. I have a bunch of ideas on how to do it too.
The basic idea will be to push a copy of the un-dubbed audio that is about to be overdubbed into a ring buffer. Then whenever a play-head approaches the record-head, fade into the un-dubbed material that was saved on the ring buffer instead. So by the time the play-head reaches the record-head’s position the mix will be 100% of the un-dubbed material in the ring buffer and 0% of the looper buffer (which has the click in it). If the play-head is going in reverse you would fade out instead.
This technique will only allow one record-head per buffer but allow multiple play-heads.