I loved the design of Rex and the confrontation at the games end is one of my favourites game moments ever. Its almost a shame he was 50 meters tall though, rendering a 1/6 replica totally implausable.
The MGS series has such a rich depth of interesting characters and the future-tech militaria art direction lends itself prefectly for a series of figures by Hot Toys.
Rex
was a monstrosity (a mechanical T-Rex) and would be, as you said, implausible in 1/6 scale. But I wonder if the USMC Ray version from Sons of Liberty might not be possible? It was a sleeker and more modular design that might be possible to replicate if it were done in sub-assemblies... arms, legs, body, and tail, which could be assembled as a kit. It would still be huge in 1/6 scale, but Hot Toys has already made two large 1/6 scale "kits" and pulled it off quite successfully. I don't know if they'd actually be able to sell enough of them to cover the costs of producing it, but can you imagine having a 1/6 Metal Gear Ray, in full USMC camouflage paint, on display in your study or living room?
Yikes!
I don't know if Medicom actually has a "lock" on the licensing for Metal Gear with Konami or not, but I somehow doubt that Medicom will ever do any more Metal Gear figures than the two from the upcoming MGS4: Guns of the Patriots game. I've been collecting Medicom stuff for a long time now, and I've found that they tend to choose just one or two characters from any given movie or game license and leave it at that. Take the movie version of Wolverine for example. They did a few Marvel "comic" versions, but those aren't the sort of figures we're talking about. Hot Toys, on the other hand, has already made seven characters from Aliens, including the alien warrior, and the freakin' power loader!!! If anyone can make all those MGS figures for us, my money's on Hot Toys!
I'm guessing Konami is free to make a licensing deal with any company they choose, and if Hot Toys approached them with a plan to make their own 1/6 scale versions of MGS characters, I think they'd be all ears. And why not???