Back in 1980, admittedly when the golf world was simpler and B&B a whole lot cheaper, I walked up to the caddie master at St. Andrews, explained my "credentials" as a former club and Tour caddie, and asked if I could loop there. He tried to dissuade me but never said "no."
It was September, the R&A club matches were about to start, and by 1 p.m. that first day they used me. I got out every day for three weeks, and it was great experience. It helped that I killed most of my mornings waiting for a loop by playing the course, going off as a single (noballot necessary) and joining others as early as possible in the a.m.
I don't know if they are as open now as they used to be, but you cannot do better than that kind of experience. I would also think with courses like Muirfioeld, Troon and Turnberry, you'd also have a chance. By the way, the big caddie companies like Caddie Masters Int'l in the U.S. are always looking for qualified caddies for their ranks and they have contracts at some pretty good places, like Whistling Straits, which also would not be a bad place to work. Or Sebonack, which is recruiting.