Releasing a snapshot of unstable definitely seems like a step backwards.
Of course, I understand the reason for suggesting it (and not wanting to
support a testing distribution for SCC). Instead, I would suggest to
porters that they base releases off Debian stable releases. When a stable
release comes out, the distribution can be (re)built for all SCC archs;
porters can then add their own packages and fixes on top of stable, as
necessary. This has the added benefit of simplifying security maintenance;
when a DSA is announced, porters can simply rebuild (or incorporate
patches, if necessary) based on the security update. Developers can still
track unstable from scc.d.o, so as to minimize the work necessary after a
stable release (architecture-specific problems can be found and fixed as
they occur in unstable, instead of suddenly popping up in a stable
release). Porters are also in charge of their own d-i images, so there's
no need to bother the RMs with it.