[Barneyparker]

The only real option you have is to rewrite in a server side stack, PHP for example. IE6 isn't going to handle rehydration for SSR, and as you've mentioned any form of virtualization is going to be far more painful than it's worth.

It's entirely doable, the big question is - do you really want to? If this is genuinely the clients fault for not providing the IE6 requirement and asking you to work with a QA team using Ubuntu, then the costs should be covered by them. Potentially though, if the client is important enough to you, you may need to make a reduced rate offer or even take the hit yourselves. Whatever you choose, seeking legal advise may be a wise move.