I'd recommend MIT for an API.

Any update on this?

The main issue is that CC BY-NC-SA is not designed for code, and as such it doesn't clarify whether the process of linking to code (in this case, Quark code) creates a derivative work - something legal seems to be rather heavily divided on. If it does, me linking to Quark's APIs automatically forces my mod, as a derivative of Quark, and thus a derivative of a CC BY-NC-SA codebase, to be licensed under the CC BY-NC-SA. Obviously, that would be somewhat limiting.

This is why BuildCraft (and other mods!) often chose to license their API separately under more liberal terms - the APIs, after all, do not really offer a competitive advantage to other mods, and they're made to be used.

There is a way to work around this - I could make a "CharsetNonCommercial" addon which links to NC-SA licensed mods, but... just for Quark? (Many other NC mods are CC BY-NC-ND, and by the above logic I cannot add support for them at all.)