I can't fit much in a messenger bag that won't fit in my pockets. At least the bags I've seen, they're all flat for paperwork.
when I played ice hockey I commuted to practice on a klr250 dual sport bike (street legal dirt bike). I used this backpack (different color but you get the point) to carry all of my gear except the stick, which I lashed to the bike. I got weird looks on the road for sure but it really wasn't an issue, so I'm not worried about carrying a few grocery bags, I just need a quick and easy way to clip them on.
I zip tied a milk crate to the cargo rack on my motobecane moped and it worked great. It's just not an easy on-off thing and I use the rack for other stuff during highway rides.
part of why I want a harness of clips is so I don't have to spend time loading a backpack. The cashier bagged it all so I just wanna clip them on and go.
I partially got the idea from those things some people have in the backs of their SUVs where they can clip grocery bags to keep them from sliding around. My old truck had a cargo net with three sections to put stuff like grocery bags. This is just the bicycle version of those.