Man I love seeing topics like this around the forums, Quake 4 was probably one of my favourite 1v1 games to date, so I really really like this idea. I'm also a big fan of Quake 4 crouch sliding as well... I did want to mention, I would be perfectly okay for something like this to be a server command to choose if the server will have advanced portals, crouch sliding, or any feature like that. Not everybody would want it!
Also lots of people seem to be making maps where they dont use the portal sprite and they just use a wall or block icon instead, to indicate that it is a portal. That could be subject to change if something like this was ever released tho.