I am leery of using JB-Weld on a bike frame for a bracket for above reasons (hard but brittle adhesion.)
I don't know how much it would cost, but if you have to have a permanent job put in place, maybe have someone with a padded vise drill holes and pop in some rivnuts, perhaps using JB-Weld around the rivnut as a poor-man's brazing after the rivnuts are put in (to keep water from seeping past the joint to rust inside the frame.) I get concerned about frame strength and possible stress fractures/splits around the joints. Are clamp based water bottle bracket mounts not an option?
As an added bonus, when your water bottle bracket wears out or goes out of style, its much easier to replace... just two bolts instead of a major frame job.
Of course a clamp like this
http://www.bikemannetwork.com/biking/p/WC3002 or
http://www.bikemannetwork.com/biking/p/ACH2OSYS/WC3050 [1] may be the best solution.
More clamp options:
http://www.bikemannetwork.com/biking/c/ACH2OSYS
(The second one I mentioned which allows two bottles to hang from the seat post looks very tempting for me... I'm glad I looked this up.)