In fact, that is one of the requirements for Perl 6 that I started neglecting some time ago. As I explain in the README file, I still can't see how the interoperability between different languages will work without a typemapping layer. This typemapping layer will be something very similar to what XS is today, except that in a higher-level language.
This means that the usual way for language interoperability in YAP6 will be the same as p5. But it doesn't mean, however, that yap6 won't be able to handle grammars. YAP6 is a port to C of the concepts created and tested by KP6. And it's in the plans the use of YAP6 as a backend to KP6, which means that it should support anything KP6 does, which includes the KP6 grammar engine.
In summary, I don't expect high-level languages like Perl 6 and Perl 5 interoperating in the high level directly, I do expect each code to be running in each interpreter interfaced by a typemapping layer. But this doesn't means that Perl 6 based sub-languages and DSLs (domain specific languages) won't be able to run inside yap6 (as long as no typemapping is needed).