Non Poly guests can watch the fireworks there if you have a dining reservation. Maybe try making a dinner reservation before the fireworks? You can hear the wishes music and see the fireworks from ohana (restaurant inside the poly) and also on the beach which also plays the music. I believe the security guards will not let you through the gates unless you have a hotel or dining reservation.
If you are staying at a different disney hotel you get free parking at all of the theme parks (that's what we always get anyways). Maybe you could park at the TTC, take the Express Monorail to the MK, then hop on the Resort monorail and get off at the poly. A lot of work, but guaranteed if you really want to see the fireworks on the beach.
Good luck!