An interesting idea, but to use a word of yester-year, this will be “inequitable” – only rich girls will marry early. Also, punishments rarely work. How about paying (kollel, for example) to yeshiva bochrim who marry older girls? I am hesitant to suggest for them to marry earlier, because most not ready for real life at the age they marry now.
Another consideration – maybe definition of the “community” is part of the problem? Do people prefer t marry within the same chasidut or “yeshiva velt”. How about going out a little bit – looking at girls who did not hit important seminaries, but might be coming from ehrliche observant families?