First, the inputs take voltages. You can see that on page 5 of the
Operator Interface Reference Guide. So no need to take apart your OI when IFI tells you what's going on there.
Second, I don't think there's been a lot of research into this area. This is mostly due to:
A. You're limited to the 100mA you can pull from the 5V pin, and probably less.
and
B. There's only so much you can do giving the RC a byte at a time using the OI.
Well. B is mostly my opinion. But I'm not really seeing what, aside from the USB chicklet, you can do with a highly custom circuit at the OI that you couldn't do with a simpler circuit and some processing or a custom circuit on the RC. All I can come up with at the moment is some sort of up-down counter to give you very precise control over the value of the byte with a 7-segment display to tell you what you're outputting. Except that's quite likely to exceed 100mA and, again, you could do it in software on the RC and the user byte.
Mostly I think the 5V, 100ma limit is just too restrictive for teams to do much fancy stuff.