I don't think you should connect the ring to a ground at all. If connected to anything, it should be to a PTT input. That way, the WinKeyer USB can control PTT and you can key the Flex without setting BreakIn. I built my own 1/8" TRS (WinKeyer) to DB15 cable (Flex AUX input). I don't have the DB15 pinout in front of me, but the tip went to CW input, the ring to PTT input and sleeve to ground. It worked on both the 6300 and 6700 (I upgraded).
By ring connector of an RCA plug do you mean the shield? If so, then the answer is "yes".
I have my WinKeyerUSB connected to my FLEX-6700 as follows:
- Center pin of RCA connector to pin 4 on ACC connector.
- Shield of RCA connector to pin 5 of ACC connector. (You may use pin 5, 6, 7, 8, or 10 of the ACC connector, they are all connected to chassis ground.)
73, Ray, K9DUR