I don't know how much this has been thought about, but I understand there is a slight dilemma regarding the way synch and ringmod works on the SID and how it doesn't fit the model of Chipsounds. I'd like to vouch for the idea that each voice simply has controls for setting an arbitrary synch/ringmod frequency, and in the case of ringmod, presumably a 'secondary waveform selector' (I'm fairly sure SID ringmod is dependant on the modulating osc shape, unlike synch which just takes it's frequency). So in a way each voice now has two oscillators, though the secondary osc would probably be switched out most of the time when not in use.
This is straying from the way things work on the SID, but given that Chipsounds is mainly a tool for making new music, and not one for playing existing music, I don't think it matters a great deal, and if we really wanted the frequency to be tracking other audible notes, it could be done with midi CC's and a bit of creativity.
That's my take on the matter, anyway, I don't know what other people think.
[Edit - another alternative would be to take the frequencies from another midi channel, perhaps?]
As a side note, I read somewhere that C64 ringmodulation isn't actually a multiplication of two waveforms, but rather some weird waveshaping or something. Information is pretty vague on the subject and there's a lot of misinfo. out there, but whatever it is, it's a freaking cool effect