That would break credit card readers, barcode scanners, and special purpose keyboards (like video workstations).

I suppose you could rely on a binary code, where the user is instructed to press any two keys to repeat the code, or if there is only one key (or it's a barcode scanner), to enter the same input multiple times, separated by long and short pauses.

Still seems like it would be difficult for the average user.

Having those devices ID themselves to the OS as a keyboard sounds like an idea where someone said "it'll be fine" rather than doing something more specific. sure, hindsight is 20/20 and all.