I don't think that the SuperCollider team has an official stance on this and I can't really speak for all developers. Legally, I think you're on the safe side as long as scsynth runs in a separate process (which is the case for Sonic Pi). However, it's probably against the intentions of James McCartney. If he had wanted scsynth to be used by non-GPL programs, he would have released it under the LGPL. But that's just my assumption. If you're in doubt, just ask him :)

also keep in mind that the gnu.org want gpl to apply as widely as possible. Generally, your approach should be fine imo