Most club race series race with numbers up to around 999, and maybe more. (There's someone at my local track who uses something like 1691)
I personally would like to see the option to have more numbers officially in GPB, sure you can put whatever number you want on your bike skin, but adding the ability to have this supported in game menus and on result sheets would be awesome.
I'm also aware that modern GP's don't use this format, so can understand the reluctance to put it in, but it would be a nice touch and shouldn't be to hard to do?