RJM,
The relevant rule is 14.2.3.2 in both the D1 and D2 Manual. Note that D1 has three versions of the rule, effective 2010, 2011, and 2012.
I believe the intent of both the D1 and D2 rule is to allow a player to graduate in spring 2011, play in organized competition, and then enroll in fall 2012 without penalty. The D2 rule almost says that, but leaves open the possibility that if the player attends a college which holds a regular academic term during the summer, he might need to enroll in the summer 2012 term.
The D1 rule previously only applied to tennis, swimming and diving. In 2010, it was generalized to all sports with a phased inroduction. The old rule wasn't well written, and in generalizing it, it has become a mess. Taken literally, and supplying the word "or" to make a complete sentence, it seems to say that if he waits until the fall of 2012 to enroll, he would have to sit out a year if he engaged in organized competition. On the other hand, he wouldn't lose a season of competition unless he waited another term to enroll. Remarkably, organized competition could mean a three-legged sack race at the county fair.
Again, I think the NCAA intended to allow a 2011 player to enroll in fall 2012 without penalty. But the D1 rule doesn't say that, and there isn't any history for sports like baseball.
BTW, you mention college summer ball. A player who has not yet enrolled in a 4 year school is a prospective student athlete, and the NCAA has started enforcing that teams which employ college coaches can't take on prospective student athletes. Mostly this applies to JC players, but it would apply to your hypothetical situation also. So that could significantly reduce the chances of finding a team.