There isn't much use for that now that the home cooked levels list actually works, I think. You shouldn't really be using Save+Run for levels that aren't yours anyway, and if they are yours, you can just attach a quick angelscript file to morph the player to spaz. Or you're welcome to run the game from command line or whatever, just not JCS's Save+Run.

I'm not so clear why Race wasn't supported online, besides a lack of interest, but the reason for Cooperative not being included is that it's significantly more technically complicated than Battle/CTF/Treasure. Player movement in JJ2 is so fast and fluid, much faster than the network prediction code can handle already, and the game just isn't equipped to try to sync every enemy and boss on top of that. That's why you can't push crates and monitors and things online, either; their positions would need to be synced across clients. You look at something like Little Big Planet that is playable online, and they had to take various measures to make that possible, like inserting a delay between pressing the jump key and having the player actually jump.