I have used a hot glue gun to do something similar a few years ago.
I bought a sheet of velcro and hot glued it to the inside of the bag (it was a photo rucksack made out of 1000d cordura). I had an Ortlieb water carrier that I wanted to use as a full on waterproof section to the bag so I hotglued the other section of velcro to this and then compressed the two sections of velcro together. If you have enough area of velcro it does provide a terrific amount of grip and can support a lot of movement and weight.
However! Be warned, I didn't try the glue on the Ortlieb first and when I put the first gloop on the material it started to melt with the heat (worked fine when I applied the glue to the velcro first though). This could be tacked in place with some sparse stitching at the main stress points. I also used this method successfully on a SA assault vest mod.
It won't win any prizes for professionalism, but it's pretty easy to do.
"Boy, I remember the first time I got shot out of a cannon"