can you enable ctrl-u in fullscreen mode?
i use it to watermark pics, different scripts/colors etc, and doing from fullscreen is the best way vs thumbs which aren't always big enough.
It's not so complicated, just use spy.exe from HotkeyP menu, perform action you need in xnview view mode (e.g. select menu > image > add watermark) and copy the command, process and window class to "hot key" window in HotkeyP - just select the line, then the buttons above.
For "add watermark" it's:
Hot key (custom): Ctrl+U
Command: Command to active window
Parameters: 338 xnview.exe XmainClass
Make sure you're in the "view mode" fulsscreen, not the browser fullscreen, go to view mode and press F11.