Who would be placing those buy orders?

I think a better starting point is a discussion of what extent the Hive Fund should all be converted to HBD, or if a significant portion should be staked as a delegations fund. That fund could then allocate non-voting resources to projects that need to create new accounts, process transactions, and otherwise allow users to use Hive. A bit like how steemit used much of its stake to create new accounts.

I think there is a good argument to be made in favor of delegations. As long as it doesn't carry with it the disincentives that the delegations to projects did on Steem where receivers cared more about min-maxing the earnings from their delegation than actually growing their dApp.