I thought GitHub issue #108 presented an interesting use case:
There is a package for LaTeX that is quite popular among users in Russia and eastern Europe — pscyr. It contains a number of fonts used by publishers in USSR, for instance, Literaturnaya. Some of these fonts were digitized and edited during 90s and their source and license status is unclear. Because of that, pscyr package is not included in any distribution.
For Texlive, installation includes copying files from archive to texmf directory and running mktexlsr && updmap-sys. Could it be possible to integrate an “external” package into tectonic?
As I replied in the issue, for the same reason that the distributions won’t include the package, I don’t think Tectonic can distribute it either. So if you really need to use the pscyr package, the best we can say is that you should unpack all of its files in the same directory as your input file. That’s clearly lame.
I’m having trouble thinking of how I want to approach these sorts of cases, but it’s becoming clearer that we need some kind of solution. We could download TeXLive packages, but I don’t believe that TeXLive archives old versions of packages, which would be essential to reproducibility.