Are there any plans to either add additional language support, or create an XML interface for added custom language support?
Specifically, I'd love to be able to use NClass for PHP and Visual Basic .NET in addition to the already supported C# and Java
Well, maybe once I'll add more language support, but the development of NClass is currently suspended, so it's a no for now.
i added support for php to nclass. currently it's beta but if you want to try it out i can send you the source code