Syn provides dynamic signal patching into SynthDef inputs: initialize or set a control to a Plug object and it will manage the bus(es) for you – that much is like JITLib somewhat, but Syn/Plug are designed to be used polyphonically just as easily as Synth is.
AFAICS it’s a quite common request, now with a neat solution, but it’s very hard for quarks to gain traction.