I think I know how to bypass the SP missions..but...
1) make the starflier a buyable ship on manhattan only and reduce the price to $1
2) Edit every single item and ship to drop the required levels down to 0
3) Unlock all the jumpgates in the initialworld.ini
4) Then, start a new game and save it as soon as you get to manhattan
5) Disable the storyline in the freelancer.ini
The reason for starting a game and saving before disabling the story is because if you disable the story first, you start where Freeport 7 was and have no way to leave...
As you can tell, this would make the game playable, you would just never gain levels...but that wouldn't matter if the required level for all equipment and ships was level 0. Then you could play basically like multiplayer, only you wouldn't have to worry about running a server and client on the same machine.
However, no. 2 is going to be the hardest part and I really don't know how many ini's you'd have to change to achieve that. Also, I don't know how to change the prices on ships yet, so if someone could help me there...