Do end users commonly want to copy programs between OSes, or will they usually just revisit the project's website and download it again? My feeling is it would be the latter.
And if we're talking about running multiple OSes on the same computer, is it common to share filesystems between those OSes? It's been a while since I ran more than one OS, but trying to do that wasn't remotely pleasant.
I think this is a huge technical achievement, and I do think there are some use cases for it here and there, but I personally don't see a big benefit over just using a CI system to automatically create a build for each OS/arch combination at release time.
I also wonder about the trade offs involved in using a different libc. As an example, when I was looking into musl, I realized that it didn't use NSS (unsurprising, since that would defeat the whole zero-dependency static linking thing), and therefore had no support for multicast DNS (well, not that NSS is required, but they also didn't, and had no plans to, integrate mDNS support of their own). I've pretty much abandoned using musl (and Alpine for containers) due to random quirks and gotchas like this that end up biting me somewhere down the line.