I don't know that any collegiate summer leagues have ever been successful. There's currently the Beach Collegiate Softball League in Myrtle Beach. I don't know anything about the quality of the league. There's no need for softball players to prove they can hit with wood. There aren't any pro scouts to get in front of. Team USA knows who they want. The National Pro Fast Pitch League has been unstable folding and restarting since the beginning 17 years ago. This year there are only four teams. Even when there were eight the teams knew who the players were they wanted. The big names used to play in attempt to jump start women's softball as a major sport.
At the pre college level softball showcasing is just as big of a deal as baseball showcasing. Cathy Aradi once wrote a softball recruiting book that could have easily been translated into baseball. It's everything we discuss on this board that's now common knowledge for anyone who has been involved.
Many college softball players hook up with top 22/23U softball teams and travel the region playing ASA tournaments on weekends just like they did in high school playing 18U.
Disclaimer on my knowledge: My daughter is four years removed from college softball and I don't follow the sport anymore. There was a time when I could list all the top college players in the country and Team USA. Even my daughter would rather watch men's sports than women's sports.