I tinkered around with the source code earlier this year trying to add this feature, and I was successful up to a point. I could make a 6GB file, but running it gave a "Not a valid win32 application" error.

I researched up about this, and it turns out the size limit for executables is 2GB, which is a limitation of the PE format, NOT nsis.

This failure resulted in my WimImage plugin being written, so as far as I'm concerned this request is closed-notaccepted.