OK, I figured out what I was doing wrong.
For anyone else who wants to do this, follow the instructions to install Cygwin and build your svn download of rockbox. (Or you could try to contact me for a copy of my saved ihp_120.bin.)
Place a copy of the Iriver original (NOT patched) firmware in the /rockbox/tools directory.
From a standard command prompt screen within Windows (Start/Programs/Accessories/Command Prompt) go to that same directory -- ...cygwin/rockbox/tools -- and issue the command (no quotes) "descramble -iriver ihp_120.hex ihp_120.bin". (If you get an error about missing cygwin1.dll [I did, but I had one botched installation of cygwin], find that file in the cygwin/bin directory and copy it to your /windows/system32.)
Copy your ihp_120.bin to your player in the root directory - the same place where you already probably have a patched ihp_120.hex and your bootloader.
Boot the player normally into Rockbox (I followed the instructions and made sure no music was loaded on boot - I just went straight to the file view of the root directory). If you haven't done this already, make sure to first boot the player with the HOLD switch on and follow the prompts to set the default to boot from disk (not RAM or ROM), then reboot.
If you don't see your ihp_120.bin file in the root directory, go into the player options using A-B/General Settings/File View/Show Files and select All.
Go back to the root directory, navigate over the file ihp_120.bin, press and hold the NAVI button then select Open With ..., then select iriver_flash, then press the NAVI button and finally press Play to start the firmware flash back to Iriver.
Now, with the patched ihp_120.hex still in your root folder (or put one there), you can still boot into Rockbox or back to the original firmware by holding the REC button when turning it on.
You can delete the ihp_120.bin from the root directory and also go back into A-B/General Settings/File View/Show Files and select Supported to hide unplayable files from view.