Where have you seen these concepts, blueskirt? I ask because I've had that idea (and unlike you I think it would be cool), but I've never seen anyone else propose it.
I've seen this idea proposed many many time back in 2000-2003 on older, now defunct, adventure games forums and chatrooms, it was pretty much the sole solution people could think of anytime such topic was brought up, and most discussions were wasted thinking about solutions for this concept's problems and technicalities, rather than considering all the alternative available (such as giving each players their own character).
You don't need to pit 8+ different players against each others in the same game to have multiplayer, more and more action games, like Splinter Cell: Chaos Theory, feature co-op multiplayer, in some games you can play the main campaign and story in co-op with one or several friends, rather than keeping the multiplayer aspect in a story less environment. And IMO that's where the answer to multiplayer adventure game is. If the idea of multiplayer adventure games had been a big dream back in the golden age of adventure games, MM, DOTT and Gobliins 2 could have very well been the games that would have gotten this dream off the ground (albeit with much less timed puzzles in the case of Gob2).
With multiple characters, it's just a matter of playtesting different ideas and implementations to find the optimal formula. Whether it is better to have a common shared inventory like in Gob2 or a separate inventory for each characters like in MM, whether is it better to keep all characters in the same room and keep the action in several small enclosed area, or to let them lose themselves all over the game area, or give each characters their own area to explore, allowing players to switch roles once the game is over, whether each characters should have different talents or give them all the same capabilities...
Back in the IF days, didn't people consider adventure games to be a spin off on RPG video games, where the focus was less about exploring maze, killing monster and leveling up and more about storytelling, interacting with your environment, solving puzzles and discussing with NPCs? If so then pen and paper RPGs that focus less on battle and more on roleplaying and puzzle solving, like we did with my old roleplaying group, are pretty much multiplayer adventure games. It can be done, I am sure of that, it's just a matter of someone actually doing it.