After a quick search, I couldn't find any direct posts regarding how one would go about cleaning/washing their messenger bag.
I guess it depends on the bag/manufacturer. So... I have a ReLoad Backpack. There's very few metal pieces, and none of them are integral to the bag's construction. Though there are numerous plastic clips that are important...
So, anybody have any personal experience with getting their bag clean?

There was a long discussion on getting the stink out of a bag here: https://www.bikeforums.net/singlespeed-fixed-gear/119266-how-do-you-de-stankify-messenger-bag.html

Not sure if that answers your question or not, but some good information regarding the cleaning of bags...


EDIT: to summarize - soak the bag in vinegar overnight, throw it in a washing machine, then hang it over a chair to dry.

Hand wash, hang to dry. Bags are bags and messenger bags are built to withstand the elements. I dunk in a tub with light detergent and agitate and then hang to dry. Vinegar is useful if your bag stinks.