pip is already considering the mechanics of making --only-binary the default (as usual, it’s not as easy as just making it the default).
This is inspired! I don’t think backtracking will work today, but it’s certainly a neat way to allow feature detection without having to change any existing metadata.
I think we’re still a little more limited than this, but we can at least do split packing (so the metadata can be read from an outer ZIP file, but the contents are packed again in something else). Pretty sure it’s too soon to assume/require that the index is serving separate metadata, or that an index will be capable of parsing the new format to extract the metadata. I think you covered this with “installing offline”, but it’s going to apply to online installs too for quite a while.