I worked somewhere that maintained their own branch of some software in this way, and they then committed the patches to git. A strange workflow for sure, but it seemed to mostly work
This submission about some mysql fork ships that fork as a couple of 15MB .patch files in git https://news.ycombinator.com/item?id=44148039 -> https://github.com/enhancedformysql/enhancedformysql/blob/fb...
I am not the target audience for such a thing and thus didn't try to apply said patch but my experience has been that I'm not going to hold my breath
I maintained a long-lived branch once that involved major refactorings, with a lot of machine translation and a lot of manual translation. Periodic rerbasing involved a lot of diff diffing, and editing patch files was often a very practical approach.