ok, the simple (but slightly incorrect) answer, is you cannot do this.
voice allocation is done by midi note, which is done in the midi handler in the parent patch.
so you need midi note to invoke a voice....
the normal way to do unison is to put the oscillator in the same patch
BUT i said, this was the simple but incorrect answer
the reason is, you are able to generate midi, and send it via the 'internal' bus, so i this way, you can create your new midi note, and it will then be sent to the subpatch and allocate a voice accordingly.
(not tried, but this should work fine)
but I don't think it would be too difficult for us to create a new subpatch type, that allow explicit routing.
(the main issue being the inlets currently go to all voices, so it would have to somehow 'stall' these and only send to the active voice ... remember we are using midi as the routing currently which is a different handler)