Well, I have...and gettin' crafty.
I decided to add some wax to my Velo-Orange Grand Cru handlebar bag to make it a little more waterproof. I haven't actually ridden in anything more than a drizzle yet, but it pays to be prepared. Plus, who doesn't like to wax stuff?
Fifteen minutes of internet research led me to purchase 4 ounces of beeswax and 1 lb of paraffin wax. One or both may be available at your local hardware store. Mixed in a ratio of 1 part beeswax to 4 parts paraffin, it makes a good wax for applying to cotton.
Some people seem to have lots of trouble applying waxing products to cotton ("had to throw away my Carhartt jacket, it was RUINED!!!" says The Internet). I'm not sure what they did differently, so here's what I did.
After heating in a jar in a water bath on the stove, I used a cheap paintbrush to apply to the cotton. The wax instantly dried when it hit the room temperature cotton. That's the white stuff on the bag below.
Then I used a hairdryer to melt the wax into the cotton and a rag to rub the wax in and take off any excess. Pretty straightforward, but time consuming. When finished, the cotton looks a little irregular, about the same as something well aged. I also did the leather accents, since beeswax and paraffin are ingredients in most leather waterproofing products. You can tell in the picture below, the leather on the left is a little darker, it got a little more wax.
I only did the front and the inside of the top. There is a plastic stiffener which is in between the inside and outside cotton layers. It runs down one side, under the bottom and up the other side, acting as a reasonable moisture barrier. And the rear pockets are covered by the top flap and don't get rained on as much because when riding, rain mostly hits the front and top.
Total time, 1 hr.
I would pay an extra $30 to have the whole exterior of the bag treated like this during the manufacturing process.
Here are some pictures after riding for 11 hours on a rainy day. Rain was light but steady for 6-8 hours and there was a lot of road spray off other riders hitting the front of the bag.
Front, completely soaked on the outside
And completely dry on the inside
Ride more, even in the rain.