All, I have received the following response from Steinberg (email details removed):
Dear Mr. XXXX,
Thank you for your interest in the UR 22!
I can confirm, that the UR 22 is working as a standalone unit aswell.
The inputs 1 and 2 are then (hard-)routed to the headphones and to the line outputs.
It is furthermore possible to operate the UR 22 with a regular USB power supply, without connecting it to a computer.
If you have any more questions, please let me know!
Tonigth the UR22 comes in from Amazon, let’s try !!