If the aim is to provide 32-bit libraries for amd64 systems then the “better” approach would be to package lib32-* versions, e.g. lib32-curl .
No extra repository or architecture is needed, just more/different packages, and those packages can be built and updated alongside their 64-bit counterparts by the same infrastructure.
This is already done for a number of packages in the Ubuntu repos, and it’s the approach taken by other distros, so it works.
If I can find time next week I might put together a PoC PPA…