Even if technically possible, it's unlikely this will be used to support any of the variants you mentioned in Debian. Both i386 and armel are effectively dead: i386 is reduced to a partial architecture only for backwards compatibility reasons, and armel has been removed entirely from development of the next release.

What you said is correct wrt. official support, but Debian also has an unofficial ports infrastructure that could be repurposed towards enabling Debian for older architecture variants.

This would allow mixing armel and softvfp ABIs, but not hard float ABIs, at least across compilation unit boundaries (that said, GCC never seems to optimize ABI bottlenecks within a compilation unit anyway)

> Could this be used to address the pesky armel vs. armel+hardfloat vs. armhf issue

No, because those are different ABIs (and a debian architecture is really an ABI)

> the issue of i486 vs. i586 vs. i686 vs. the many varieties of MMX and SSE extensions for 32-bit?

It could be used for this but it's about 15 years too late to care surely?

> (There is some older text in the Debian Wiki https://wiki.debian.org/ArchitectureVariants but it's not clear if it's directly related to this effort)

Yeah that is a previous version of the same design. I need to get back to talking to Debian folks about this.