I don't do mirroring - I am more of a RAID kind of guy. However isn't mirroring done by ASP? If so, then you assign which ASP a PF belongs to by the library it is in. So, if your LF's are in the same library as your PF's, and that library is on an ASP that is mirrored then your LF's should be mirrored. Make sense?
CRTLIB LIB(MYLIB) ASP(1)