That's an interesting idea, though I'm not sure I can see how it would be valuable in practice. Do you have systems where the hardware setup is identitical but the ARM CPU is different?
Nope. The arch number specifies what kind of arm-board the kernel is booting on. One kernel binary is in my case able to handle 3 different versions of boards (and its hard or dangerous to find out by probing the board). The number is used to tell the kernel about how irq-lines etc are wired on the board. Perhaps the name 'arch' is a bad choice, perhaps "arm-mach" is better.
Yes, I understand that. It is also important that the board setup be compatible which is the responsibility of uboot. Which seems to be true in your case. None of the Arm chips I've used have this feature. While they can be dynamically detected without danger, it is just as easy to make separate u-boot builds for each.
Which chips are you using?