For my purposes, it is the lvalueness associated with PVLVs that is of most interest.

All of magics with a setter in their vtable have lvalueness. There's nothing special about PERL_MAGIC_substr, and the use of PVLV is not a requirement.

Note that returning a PVLV or something magical doesn't make your function an lvalue function. That's a compile-time determination indicated by the :lvalue attribute.

Are you really asking about making an lvalue function?