Extending Properties language

Hello,

I'm making plugin for our internal framework.
In our framework we use extended properties files - they have a hierarchy(child can have many parents). Of course child inherit all properties from parents.
I need replace meaning(PSI) of some(not all - only in certain scope) ".properties" files - similar like Maven or Ant or Spring plugins doing for their xml configs.
I looked at Maven and Ant plugins, but they use DOM "sugar" extension points provided by the platform.
I make my FileType and Language, but I don't konow how to register language and annotate .properties files with this language only in certain scope.
Standard FileTypeFactory extension point does not suits me, because FileTypeConsumer provides only name of a file, without file path(which I need to distinguish nls files - our custom properties).
I also tried to use LanguageSubstitutor and LanguageProvider extension points - my implementation of first one is not called, and second interface is not used anywhere in IDEA sources.

The best variant for me is to extend Properties language so that nls-files will act as normal properties files, but if I need I'll be able to get
extended PSI(for example to make custom completion contributor). But I don't want to modify Properties plugin sources - I want make plugin on top of Properties plugin.
Can you suggest starting points or examples for this?

Thanks in advance.