good news/bad news:
Bad news is the fix that fixed the shields similar separate usb device for back doesn't work with the gpd, and without a unit Radius can't get to the bottom of it.
Good news is that the workaround is simple, and provides full functionality without the loss of input when using the back button:
- in the settings input menu, disable use back as menu toggle
- in the same menu, enable menu toggle combo of your preference (i like l3+r3)
- in the input hotkey config bind menu toggle to vol+
vol+ will not work as a menu toggle, but effectively clears the automapping of back. Now when you toggle the menu back and forth with your hotkey combo, it will work fine, and will not "break" the game controls when returning to the game, especially when tabbing in and out of retroarch. Going to continue working on a fix with radius, he's looking to score a unit cheap if possible.