Technically even England assign squad-numbers that way, although it's on a game-by-game basis.
I'm sure that there have been the odd occasion where an injury in the warm-up have caused England to start with an odd-number on the pitch as opposed to 1-11.
Most recently in Wayne Rooney's international swan-song he started on the bench with #10 I think?
Would be easy to do, just have a tick-box if you wanted to use squad-numbers in your options menu, or have them 'auto-assign' 1-11 for the starting - XI on the selection screen and you can make changes after?