Frankly, I'm surprised this has never been done before. I'm not doing it or anything, I was just thinking of some ways it could be feasible since Ive gotten into series recently. (I got the Essential Collection for PS2 after seeing it for $7 at my friend's store. It was in good condition too.)

You could use rips from the MSX version of the older games, or maybe even the GBC game. Realistic MGS enemies would be easier with FFCs. It was even technically possible pre-FFC via loads of combo cycling. Here is what I'm thinking: Have a combo (guard) move along a set path. For a few tiles in front of him, he can see (since the guards in the series don't seem to have the best eyesight apparently). His sight line can be direct warp combos to a screen with a brief "alert" animation then to a screen where regular ZC enemies appear were the guard was. Or the entire guard combo can be long enough to accommodate his sight line, and be made into a Direct warp. I was thinking Stalfos 2's for generic guards. Some tweaking would need to be done to get the enemies to appear right where they were before, and if you wanted to make it really accurate, they would have to get on your dick. I can't think of one enemy in ZC that actually chases you, and just you. Ropes are the closest thing, I guess.

The codec would require scripting, most likely. Either that, or an elaborate use of side warps which I'm more in favor of... I was actually thinking about it this way: If it was possible, have a custom item that takes you to a map (maybe even the whistle could work in a pinch) and this map would be the codec screen. You have a warp on each side of 'Snlink', and going left or right, or even up or down would change the channel. Pressing a third direction would go a calling screen. If there was no one there, you'd be warped back to the original screen. This is just an idea though, and it would be hard to get different stuff in there. You could have different codec maps for different points in the game. You could also have the illusion of different things said by having one codec explaining something in a cutscene then, talking to them normally says the thing they will repeat each time. For example, Otacon may explain a C4 when you find one, but if you talk to him afterwords, he says something completely different. Maybe even a different map can be used for codecs post C4 to give an even wider range of things said. Also, maybe using the codec on certain screens would trigger certain things. It might be easier to have a case-by-case codec appear only when necessary.

Ah.. Now wouldn't this be neat?