Back-filling prometheus (and related system) metrics?
Answering my own question (after a few days of research).
I have yet to test this but https://github.com/Cleafy/promqueen seems to be designed for this purpose:

PromQueen made possible to record prometheus metrics offline. PromQueen can, &gt; therefore, backfill the recorded data inside a native prometheus database.
PromQueen is composed of two primary tools:

promrec tapes the metrics in a specified output file.
promplay backfills the prometheus database from scratch.


It doesn't have a lot of contributors but it comes from another POC repo and it was updated in the last 6 months, so I suspect it'll work fine.
In any case, it proves that you can plausibly back-fill prometheus, so it's not impossible.
Roadmap - Prometheus
You can also find this in the prometheus road-map, so it's clearly slated to be built-in eventually.
https://prometheus.io/docs/introduction/roadmap/

Backfill time series
Backfilling will permit bulk loads of data in the past. This will allow for retroactive rule evaluations, and transferring old data from other monitoring systems.