I think code regions probably wouldn’t be supported in method bodies since they, as you put it, would be a (somewhat) “ridiculous feature” – However, in C#, this does work, at least in VS 2008 and VS 2010 – just not in VB.NET.
That being said, I would avoid it. Putting regions within a method body would just leads to people making larger methods (since that’s the only time when it would be worthwhile), which is something that should be avoided, not encouraged.
If your code:
defies simple refactoring such as method extraction
I would focus, instead, on doing “complex” refactoring (or whatever it takes) to try to break up those methods. There is no way your “four or five hundred lines” long methods are at all maintainable in their current state.
Personally, I would leave them causing “pain” – make it obvious that they need work, right front and center, until you can break them up and refactor out portions.