I mean yes, you can add the flag but then you need to repeat signing rounds to update transactions since it's 2of2.
Discussion
Well, read the article. As I explain that's not hard to do in an automated system.
Ah, pre-signing various variants. I don't like it much and it'd be nice to try to figure out if it can be solved differently but from first glance it looks like it's at least better than the current situation.
You know, I've actually had people send me pre-signed txs at multiple fee rates before when they wanted to transfer me large amounts of ₿ and wanted to give me the option of choosing what fee rate to pay.
It's easily automated; the code can do all the hard work behind the scenes.