Ah, I misread your OP - the only part I really planned to support is "this package requires building from source, and may run arbitrary code now (not just after installation) to do so; are you sure you want to proceed?". Although the other stuff certainly seems worthwhile - I think it would be easier to plug into my design than into pip. Especially since I'm explicitly creating an internal API first and wrapping a separate CLI package around that.

The project is still in quite early development stages, and not yet really usable for anything - I've been struggling to make myself sit down and implement even simple things, just personal issues. But the repository is at https://github.com/zahlman/paper and I am planning a Show HN when it seems appropriate. Hopefully the existing code at least gives an idea of the top-level design. I also described it a bit in https://news.ycombinator.com/item?id=43825508 .

I've also written a few posts on my blog (https://zahlman.github.io) about design problems with pip, and going forward I'll be continuing that, along with giving a basic overview of the PAPER design and explaining how it addresses the problems I've identified.