Leasing an HD subchannel is handled privately. Having recently brokered one, I can assure you there's still plenty of paperwork involved, mostly for the legal protection of the actual licensee. You have to account for who runs the legal ID, how the programming is to be provided, how programming standards are to be maintained (especially when the programming isn't in English), and so on. It's most akin to a time brokerage agreement that you'd use for a leased-time station, because that's essentially what it is.