Thanks. I was aware of Statistics::R but had not seen R::YapRI before. However, as you note, they go in the wrong direction for my use case.
A generic API is a good target. One could even access such a service from different versions of Perl, which is a not entirely facetious statement as the dependency tree for the client will be much smaller than for the engine and maybe there are cases where the Perl cannot be updated but a packed exe can be installed that takes advantage of newer features. That case is somewhat tangential to this thread, though.