I’ll try and answer my own question.
According to CFL.ca, there have been seven Grey Cups (out of 94) that were decided by a single point. The last one was the 76th Cup in 1988 when the underdog Bombers defeated the Lions 22-21. There was one rouge in this game scored by B.C…
The 71st Grey Cup in 1983 saw the Argos come from behind to beat B.C 18-17 (in B.C.). Hank (The Shank) Illesic (who was playing in his 6th consecutive Grey Cup victory) scored two rouges in the third quarter (he missed three of four field goal attempts) but the Argos went on to win for the first time since 1954 when they scored nine points in the fourth quarter to complete the come back.
In the 63rd final in 1975 in Calgary, Edmonton defeated Montréal 9-8. There were no touch downs in this game and the only rouges (2) were scored by the Als’ Don Sweet.
The Arogos beat the Bombers 10-9 in 1947 at Varsity Stadium in the 35th Grey Cup when Joe Krol scored four rouges in the game, the last one on the final play FTW.
In 1944 (32nd GC) Navy beat Hamilton 7-6. Dutch Davey kicked the winning rouge.
In 1939 the Bombers defeated the Rough Riders 8-7 with a rouge.
In 1937 the Argos beat the Bombers 4-3 with a rouge on a 70-yard punt.
1933 Toronto beat Sarnia 4-3 with a rouge on a frozen field.
So the answer to my question is YES. There have been Grey Cups decided by a rouge. Five of them in fact, but NONE since 1947. I’m not counting Toronto’s 1983 win because half their points were scored in the fourth quarter and none of those points were because of a rouge. Either way, a winning rouge in a Grey Cup is extremely rare, especially since 1947. It has never happened in the CFL, which was formed in 1956 (as the CFC).