I enjoyed learning everything for the article, but it turns out I might not have had to do nearly as much gruntwork.
Thanks to Aidan Fitzpatrick of Reincubate for telling me that not only does their iPhone Backup Extractor utility have a way to modify a backup such that you can change data and restore it to an iPhone, they’ve written instructions explicitly for Momento.
It turns out my hunch about backups having checksums was correct, but apparently, iPhone Backup Extractor can update those checksums to match the new data. I’ll give it a try soon.
Their support team has written a bunch of similar how-to guides that are well worth checking out if you’re having issues.