C2 uses node webkit for exe, and another game using node webkit was greenlit for steam, so yes, getting a C2 game on steam is possible.
As for the 1 gb size, to be honest, I'm not sure If C2 is currently suitable for such a thing. I have no idea what node webkit would try to do memory wise, but I think what might happen is node webkit would try to load everything into ram at the start at once, resulting in long load/preview times at the start and probably using more memory than a lot of computers have (1gb of assets is way larger when decompressed), even if layout by layout loading keeps the VRAM usage low by only using some of those assets at a time. Then again, it might handle it no problem. I really have no idea. The only way to find out is to try it yourself, as I'm not sure anyone has tried such a thing yet.
If it doesnt work well, you could use the node webkit plugin to load graphics from disk when you need them, but this is a cumbersome process if you're talking about loading animations, as it is not automated at all and you have to code it all yourself. It works fine for background images though.
The only other idea I have is to break it up into multiple 'pages' - when going from one section of the game to another, using the browser plugin to browse to another page. That could also be cumbersome to do, though, and could also be pointless if node webkit can actually handle it automatically.
If you wanted to test it, you could make a tiny slice of the game without the game logic - by that, I mean import an approximate amount of graphics into the free version (not the same image over and over, C2 has deduplication methods so it wouldn't be an accurate test), but not all into one layout - 1 gb of compressed graphics will be way more than 1 gb when it gets decompressed into VRAM, and would be more than most, if not almost all graphics cards could handle at once. So, have multiple 'objects' layouts. Then run it in chrome and watch the memory use to see what happens.
Actually, come to think of it, you might also want to ask on the node webkit message board. They might know what would happen.