TCWG will introduce the concept of a 'release candidate source archive' and 'release candidate binary toolchain archive' six weeks before the projected release date of the Quarterly Binary Toolchain Release. For example, the 2015.05 Quarterly Binary Toolchain and Quarterly Source Archive will be based upon the 2015.04 source archive snapshot.
Release candidate source and binary archives will be posted to snapshots.linaro.org.
So the 'release candidate binary toolchain archive' will be in the form of source, or of binary tarball? My guess would be that, if binary toolchain users are going to look at a prerelease at all, they'll only do so if it's already built for them.