I'm not feeling it. The very few times something like that would be convenient would be offset by the security risks.
Well, physical access pwns anyway.
The problem is that a USB drive enclosure goes: USB port -> bridge chip -> SATA controller -> HDD.
A computer goes Chipset -> SATA controller -> HDD and Chipset -> USB controller -> USB port.
So getting a computer to work as a USB target disk would require some rejiggering of the chipset (which would presumably make it not be "proper.") Apple got away with it because, well, they're Apple and they can work around their own kludges and say "screw you, third-party developers."
I suppose, if you had an HDD that was, internally, connected to the USB bus, you could give the internal "hub" a second uplink port accessible from the outside of the machine. You'd have to wire it so the port was only active if the computer was shut down, though. I'm guessing that trying to address the same hard drive from two different USB controllers would not work well.