Hey, I sorta did this. Lazy substitution, at least, not lazy build. Plus a bunch of tricks to speed up downloads.

https://github.com/dnr/styx/

Lazy build sounds tricky: you don't know the contents of the package before you build it, so you don't even know what to symlink into /run/current-system/sw. I guess you'd have to have some kind of wrapper. Maybe similar to comma.

I just solved that part by setting up CI for most of my system config (integrated with the above).