Unless you found something I am unaware of, this is currently not possible. I do have an item on the to-do list to add one more "condition" to add-ons and rebuys, where a formula could be used to specify the conditions. That would allow your request to be done using a formula like:
chipStack < (defaultBuyinChips * .5)
No, I had thought that this would be a safeguard to stop people rebuying if they were above the specified number of chips set at the onset. But then realised that their would be no way that the soft ware would know how many chips a player had left in play. Unless the chip count was updated all the time - which would just make more work.
Would be easy enough to make a message appear every so often that told players what the rebuy conditions were.
That formula, where would that go?