NSRT's original intent (I read this in a forum post by Nach) was to convert ROMs to a compressed proprietary format which they would then charge everyone money to use once it caught on. If that's no longer happening, wonderful.
Citation? There certainly isn't anything in the NSRT documentation that suggests anything like that. If Nach were trying to push a proprietary compression format, then why does NSRT support several different compression types? And if that were the original intent of NSRT, it certainly failed miserably because I never saw an SNES ROM that didn't use a standard compression format such as ZIP (and NSRT has been around for years).
That said, extending headers is still the wrong way. Just look at the title of this thread.
Frankly, I don't think this XML idea will succeed at replacing INES (as much as I'd love it to, I doubt it'll happen). What I like about NES 2.0 is its backwards compatibility, which is the primary reason why UNIF failed. A new format will not catch on unless it works with existing emulators and tools.
I like the idea being discussed here, and I sincerely hope it succeeds. At the same time, I don't think we shouldn't also try to help people fix their ROMs so that they will work in emulators that haven't been updated to use the XML data.