[StevenCain]

One word of caution on pi side. Pi can only except 3.3v signals . Most definitely going to need level shifting.GPIO and uart.
How about a FTDI cable between pi USB and use TTL end (seems that everyone is thinking roboRio is TTL at expansion connection - I would want to check this before proceeding) and connect other end to expansion connection. Make sure Tx connected to Rx. And Rx connected to Tx,
Communication flow will have to be handled with xon or xoff (or simply put well thought out timing delays in the code) will probably want to flush the buffer at start of serial initiation. Both processors are capable of multitasking so any lag in image processing may cause some difficulty for the pi.