> 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.