Only problem is that the UART is only going to be used to feed the fluke. And the 9100 is a single wire interface so no handshake. The question will be just how accurate I can keep the timing on the output in software, and how sensitive the receiver in the 9100 is to timing. With Chris's help I found an app note on Atmels site about writing a software UART, it disables interrupts while transmitting, so no keyboard input can come in while it is sending a character.
Going to have to figure out if that is going to cause too much trouble.
Its going to take some playing with I am sure but software is my thing so I should be able to come up with some adaptation.
I've used the larger Atmel ATmega range a fair bit, and a lot of other micro's (I do embedded development), and I'd recommend choosing the cheapest part with the hardware UART built in, for this kind of project. It will save you some work, be portable to other processors easily, and have absolutely no issues with lost keypresses, weird interrupt bugs, and buffering storage of multiple keypresss or codes coming from the keyboard (either fast typing or multi-byte key commands). In the long run to make a low volume it should work out a lot easier... Best regards, Marc