Multiplayer/Co-Op & Frictional Games? NO!!!

[Apjjm]

It shouldn't matter what the did to the rendering engine, since that's just the display, and (usually) agnostic to the underpinning game client/server code. On the other hand, I'd be able to theorize a lot more intelligently about this if I was 1. a programmer and 2. had looked at HPL1.

I was not talking about their rendering engine; I was talking about what they would have to render, which in this case would be a model for the player in all his different states. That isn't as easy as just ripping the player position data from memory and distributing it.
Furthermore, they would have to synchronise the physics engine, except the player's bodies, which would require some further awkward changes.
In fact, the only way this could ever work is to de-localise the physics, enemies, etc onto a server. Then connecting to it with clients which took info about physics state changes, enemies and other stuff, then rendered them. You could use a p2p model for player-state comms, but you would still need to most likely extend the game with a dummy player class which rendered a model with the appropriate animations. This server would also need to be able to handle multiple physics worlds and simulations simultaneously. Not going to see that happen unless somebody has the source code. Even then it would most likely be a huge departure from the current game.