JB weld ? on the seat post? all that JB weld is,is just a metal filled epoxy it will glue and fill but it will take the place of welding the seat post?answer to that NOPE! You Will be polishing your hinney on the rear tire!!! Smell bacon frying anyone???
You could get a better result by using a fiber glass cloth with epoxy. the glass cloth will give it some strength. I wouldn't do the cloth glass thing though unless I made it all out of glass and cloth solid core of glass and cloth it would be the same as a glass landing gear used on airplane, but for a few seconds with a welder you'd be farther head welding it.
I like your seat post idea using what you have laying around. I don't know what the other guys weigh or do on their bikes one did said he weighs 300 lbs. Your seat isn't taking all the weight of you as your feet are on the pedals. when I'm riding I put most of my weight on the pedals. Especially if I see a bump coming.
Would you all freak out when I cut up a handle bar for an exhaust pipe header?
Oh old lawn mower handle bars make good header pipes.