Olivier asked me after reading my HUNT post if there is an option to get Invasion running on Open Arena. Besides the missing textures, which will hopefully be only a matter of time, exact replacements of Q3A playermodels won't be created. Unlike HUNT in Invasion, playermodels are fixed to the chosen armor class or alien race and are hard coded into the game logic.
On Open Arena it runs actually with default models at least if you enable cg_forcemodel and only the missing textures are a problem. If you are really into getting it working just get the sourcecode from the Invasion website and change the playermodels yourself. You will just miss a few nice skins for Orbb, Klesk and others.
Comparing its coop qualities I enjoy the fast gameplay, a nice HUD, and great weapons (no Q3A guns are used). A real Alien feeling emerges when playing Xenomorph and running across the ceiling, being the death from above. Too bad the places where eggs have to be destroyed are always the same so provided maps, which are great, get pretty repetitive. Anyway for me it is entertaining and I would prefer it over HUNT "Save the Universe". On the other hand the latter offers tons of randomized stuff you can never predict and especially "Escape from Hell" is my favourite, when it comes to coop.
Olivier did you check Tremolous and an Invasion mod for it? Maybe you will get what you are looking for. I have not tried any of these yet but I remember I had good times playing AvP1+2 multiplayer a long time ago.
