There are several different currents at work for summer ball. S is a rising soph and we believed that if you did not find a summer placement by the end of September, you would not be playing summer ball. Nothing could have been further from the truth!
There are several distinct "signing seasons." The first is the fall. If the player has a distinct idea on where he wants to play, then this is the time to work with your coach/contact in trying to get that desired team/league. Before committing understand whether the placement if for the entire summer or a "temporary" contract. Understand the difference!
After the fall period many summer teams will still have holes which need to be filled. Note that certain positions are always in demand -- pitching and catching are premium positions which come to mind.
As the collegitate spring season progresses, holes on summer rosters appear (players may be injured during spring) and disappear (a previously unsigned player has a great start and is signed) and rosters are really in flux at the margins. If you have a proactive coach with good summer contacts, he will be aware of these opportunities.
As the college season winds down, summer rosters remain in flux as college teams begin post-season play. Post-season play intrudes into the beginning of the summer season. This leaves holes in rosters which are available on a "temporary" basis. Players signed to these temporary contracts get playing time and can earn a permanent spot with their performance (although occasionally, even a great performance isn't enough because the roster is already permanently committed. In those cases the summer team will often help in placing the player with another team in the league.).
As the college post-season progresses, players get hurt, worn-out and have had enough of baseball. Then permanent spots become avilable on summer teams that had signed those players. Once more, pitchers and catchers seem to be in demand. So, now players on temporary contracts are offered permanent spots and even more spots open up.
As the summer grinds on (e.g., the NWL plays 70 games in 76 days) players drop like flies from injury, innings limits, exhaustion, etc., and teams really begin to scramble. On S's team we had players arriving after their existing summer leagues had finished!
So, the moral of this dissertation is that there are lots of spots in lots of leagues. These spots are not all filled by the end of fall. And many spots thought by the summer teams as having been filled open up unexpectedly. Your coach is the best contact for spots until the end of your spring season; late in the summer, your teammates playing on summer teams are even better as they can speak to their summer team manager about you.