If I remember correctly, on the blog post they did mention an option coming soon that allows a pot to be created where interest would be applied to it…
Quote from the post:
We don’t pay interest on your main balance and don’t plan to start — for most people, it only adds up to a few pennies and isn’t worth worrying about. However for people with money in Pots, it seems weird there isn’t any interest paid there. Within the next 3 months, we’ll let you create a Pot that’ll pay a competitive rate of interest. More details nearer the time!
On the sneak peeks for iOS too there is a rules for pots option coming soon thay could in theory be used for your purpose of: if balance is positive *ignore overdraft, then move to X pot.