The magic of words ... we transform young trees into obstructions. (Old trees, however, just stand around and complain at the special treatment given the youth. "It wasn't like that in my day, no sir!" groused the stately oak.)
The blueprint for the modern unplayable rule came from:
1941 USGA. The player is the sole judge as to whether his ball is unplayable. It may be declared unplayable at any place on the course, except in a water hazard or in casual water.
1947 the USGA said a ball could be declared anywhere on the course. In stroke play a ball may be lifted from any place except a water hazard, penalty one stroke.
1950. Unplayable is...
That Decision applies to a single stroke on a single hole. An explicit agreement to disregard a Rule, for let's say an entire round, isn't permitted. (See Wendy's posts. She has, in hand, a ruling from the USGA forbidding such.)
Wendy, do you think that a "tacit" agreement such as the casual statement in the locker room such as, "Everyone in our group of 30 players on Tuesdays and Fridays seems to play ready golf," might be acceptable? I believe that the USGA wouldn't approve of the "explicit" statement on the first tee by the guy handing out the scorecards saying, "Hey, everybody, you're all to play ready golf throughout today's round." ps I've read your position on this issue on another site...