Hi David,

The realtime folks are in Korea now. I don't know that there is a document to describe what you want done. But if you're really itching, check out what gets done with the Mask input in the RT options plugin.
Look at RT_get_mask_aves function inside plug_realtime.c . It looks like (I've just looked at that code for the first time) this is where the averaging of incoming data over an ROI happens. Then the average is sent through a tcp socket (see line 1901 in plug_realtime.c).
It looks like this average (preceded by the motion registration parameters) can be captured by serial_helper and output to a serial port.
I recommend you setup a toy example using rtfeedme, make use of the mask option (and motion parameter graphing) in RT options plugin interface, run serial_helper to monitor the plugin's output and see what you get.

Reading material in addition to .c file from above.

serial_helper -help
rtfeedme

cheers,
-z