I posted Nix and IPFS · Issue #859 · NixOS/nix · GitHub too, but per Building the technology needed to bridge the Software Heritage archive – Software Heritage we started work on the IPFS SWH bridge.
The idea with the prior IPFS Nix work was the IPFS should be a substitutor, not method of fetching. So you are always free to write down a “primary” URL to fetch from, and as long as you secure the fixed output derivation IPFS can act as an alternate way of getting it. SWH is already the archive of last resort for many things, so this puts a bunch of useful stuff on IPFS right out of the gate.