Back in the old days, I trained for Olympic distance triathlons by training for them at 120% of distance. If the swim was 1.5 k, I would train for 1.8 k. A 40 k bike became 48 k, and so on. The theory was on race day, the race seemed short, and I would say that I went from top 50% to top 33% in my age bracket. You still need talent, but I would finish feeling fresh and recovery was easier.
The complexity in golf is it would be an extra 3 or 4 holes. But you have to treat it like a 22 hole match, and not 18+4. Set par at 88 for 22 holes and play that. Obviously that adds in facility and partner issues to deal with. At Yale, I could play an afternoon round and then team up with some late day players to play the first 4 holes after coming off 18. The 4th green is pretty close to the clubhouse.
My guess is it would take at least 5+ rounds for the mentality to shift. Yes, you could just do 27 holes, but that could be a grind physically, and I think adding 4 holes is better than "playing an extra 9".