I’ve discovered a simple mod for MGS2 PC that allows you to play the Tanker chapter as alternate characters. Here, I use it to play as Solidus Snake, perfect clone of Big Boss and the endgame antagonist.
The mod also includes a package to use all the plant weapons (except the HF blade, sadly). It was produced by the now-defunct RedCode Interactive.
Demonstration of the mod:
Mod files: tinyurl
Mod file mirror: mirror
To use the mod, unzip and point the installer at whatever directory holds your MGS2 installation. This mod tool replaces files, so you might want to backup your MGS2 folder first.
You can play using several models, including Fortune, MGS1 Ocelot, and a Gurlukovich Commando - but Solidus remains my personal favourite. Sadly there seems to be no equivalent mod for the Plant chapter.