There is Synth.basicNew which creates a client side Synth object without creating a new node on the server. If you provide the right nodeID, all messages (like .free or .release or .set) will be sent to the right node. I’m not really sure about how necessary it is to provide the right SynthDef name as well though…
Anyway, in this topic I also posted some code that gets a reference (a client side Node object) for every active node on the server. Maybe it could be useful to you!