The Dell MBR for a HPA-based MediaDirect secretly stores a fifth partition table entry for the MD partition. When you press the MD button it quietly swaps that entry into the fourth slot of the real partition table, issues an IDE SETMAX command to the hard disk's onboard controller to expose the HPA, then boots the temporarily exposed partition. Since it's a secret part of the Dell MBR, tools like Partition Magic and others don't know it's there, so it shouldn't get removed by the normal disk mgmt tasks and that "fifth entry" will still be there after rebooting.
When you turn the computer on normally, IIRC the boot code checks that secret fifth entry, makes sure its HPA-hidden, then boots the regular 'active' partition. IOW, even if the Feature Tool works, I think the next time it boots the Dell MBR may discover its been unhidden and rehide it. I'll revisit the disassembled code again to recheck if I'm remembering that right.
If that's the case, that means Feature Tool alone wouldn't be enough to reclaim the space. You'd have to use Feature Tool and also alter or overwrite the Dell MBR. (I wonder if the MD repair utility would take care of that with a new MBR?)