Very interesting idea. So there'd be an internal liquidity pool and you can park your money there for a yield? And the liquidity pool is at the first-layer blockchain level?
I am also wondering if the 10% APR is actually a higher APY. From what I understood, the HBD interest is paid once a month (provided there is an incoming or outgoing HBD savings transaction) so there is compounding happening. So the APY might be more than 10%. If so, we're probably doing it wrong by advertising the APR.
By the way, there is no 3.5 waiting period when converting in order to get your HBD. You convert the Hive to HBD, then half the Hive is held and Hive is instantly converted to HBD. Then after 3.5 days you get your remaining Hive based on the median Hive price during the 3.5 days. So there's instant liquidity but the actual conversion ratio is determined over 3.5 days.