I want to control my PF motors with the RCX via the RCXs included IR. I've seen it done before, but haven't found any code on it. After looking at this document I believe that I could write my own driver. However, I'm at a loss as to what command will allow the RCX to transmit that kind of data. I see a command "SerialData[]" but when I mouse over it, it just says "Advanced Feature", and it's not in the help file. Can anyone help me?

The FW on the RCX would need to be made specifically for that task. I'm not familiar with the ROBOTC RCX FW, but I know that the Lego FW doesn't support that. Using the Lego FW, it's also impossible to have control over any low-level IR related stuff (and the higher level IR data is extremely limited).

I've read some more about this, and it seems that it's only been accomplished with BrickOS, which I believe is the closest you get to running natively on the processor.

I've contacted Laurens Valk, he's the one I saw doing it, so I'll see what he says.