So yeah basically me and another guy we've been converting lots of alien swarm stuff to be used by the asw_player which is mostly made of stuff from the baseplayer.
It turns out that changing all references to marines, to player references, works pretty well. There are some place where you'll probably need to add marine functions to the player, or improvise something, though. We got most weapons to be used by the player, however view models, anims, and weapon timing will need some work.
Here a short vid of it so far:
You can see that there seems to be a problem with the way the rate of fire is handled. I think its one of the side effects of having two competing systems running one over another... The asw weapons are basically basecombatweapons with additional function handling reloading, firing and all, speciafically for marines... Like in the asw_weapon class you have ASW_Reload in addition to the CBaseCombatWeapon's Reload...
We also fixed the camera's default orientation by modifying the newmapsetting.cfg file, which setconvar values at each new map starts, it took a while to figure out the culprit!