I like the idea. Sure I like more the collateral to create HBD :). For many reasons like "I want to buy a car" or something more expensive, it would more work as a currency. But I think you already know.
24h would be IMO better for conversion. Because of 5% I don't see something crazy happens. Maybe if HBD goes to 3$ or more.
Besides that, what if we build something like a Blackbox / return mechanic.
Send Hive to the Blackbox -----> auto conversion ----> get more hive back -----> send a return to the user. And the other way. It would generate some passive income streams and make HDB more stable. Like the DAO does. Hive to HDB to more Hive back to DAO.
Would be something like DEFI and I can imagine it should be doable.
It would be working the same as the proposal @smooth already does.
Would be maybe worth thinking about.
And after something expand to loans and collaterals on Blockchain level :)