The U.S. could launch massive day raids and needed something to organize everything. You're talking about hundreds of aircraft arriving on target at the same time or slightly seperated.
I have never heard of anybody else using formation aircraft. The British bombed at night and didn't put up the numbers the U.S. did, so would they even need a guide aircraft that they couldn't see?
As far as the invasion stripes designs went, I imagine that any reference to any kind of special markings was a tightly kept secret and only the agreed upon design would have survived for even a short time. It's possible that once everyone who needed to know was told, the paperwork was destroyed. Details were held in strictest secrecy and the markings were only applied at the last minute using anything they had on hand, paint brushes, mops, brooms and so on. If you look at some of the pictures you can see they weren't the neatest paint job applied.