HP DriveLock, according to the HP User Guides or Maintenance and Service Guides I've read, will present a single password-protected drive for unlocking during the BIOS post. If the boot drive is not password-protected but a data drive is, then it will request the user password for the data drive. I haven't seen any reference as to whether this process treats an eSATA data drive differently. If the password for the data drive isn't entered correctly then the BIOS proceeds to boot without unlocking it.
After the BIOS post, HP DriveLock won't offer to unlock a drive.
It's an interesting reference you've found regarding the Intel RST feature for unlocking a drive. However, I suspect the bit6 HDDLK register is part of the BIOS programming, and can only be modified by modifying the BIOS itself. Someone with knowledge about BIOS programming (Pit, perhaps?) should be able to tell you more.
I’ve actually tried using a password disabled internally mounted boot disk together with a password enabled data disk connected to the eSATA port. My HP EliteBook 8460p doesn’t ask for the data disk’s password at boot, and Intel’s RST Application marks the drive as locked in Windows.
I’ve also tried the same configuration as above but with both disks password enabled using the same password. The data disk is unfortunately still locked according to Intel’s RST Application.