Here's an easier solution (for me at least).
1. Put SD memory card in printer slot.
2. Press Scan button on all-in-one.
3. Select SD card as save destination
4. Press OK.
5. Wait until SD card appears on Mac desktop and drag scan file to desktop
6. REMEMBER to NEVER buy another HP all-in-one after HP cynically made older models obsolescent!

I have a DeskJet 970c with a duplex component.
With Leopard the duplex function ceased to function, and continues to fail.
As you noted, HP is no help.
Gutenprint and Foomatic do not provide the duplex function.
I resent the attempt to force me to throw away a good printer.
It is ecologically, and economically (at least from my point of view) unsound.
I will never purchase an HP product again.
What brand(s) does the readership recommend?
Or - does anyone have a solution?

sadly, a lot of manufacturers get up to these shenanigans. My Canonscan is unsupported in 10.5+. I dual boot ubuntu for when I need to use it.