Sending to seminaries is not voluntary as the girls believe b’emuna shleima, influenced by their teachers and wise peers, that not going there will lead them to a life without friends and shidduchim.
Sherut leumi could be a good idea and could be started as a volunteer service, encouraged by chachomim and organized in a proper way. Surely, there are a multitudes of way to do chesed that satisfy both charedi and medina requirements.