The way I understand you are doing it you will have a bunch of little pieces of metal showing where the holes are on the inside of the tub??
If that is acceptable to you it will work fine until you stress the bond somehow, either thru vibration or impact. First the filler will crack at the hole, then the patch will fall off. It might take days, or years to get to that point.
I have seen some bolt holes filled with bolts, then painted. It is a rustic look, but better than holes in most cases. �SOme guys use stainless or chrome bolts like bumper bolts or small similarly shaped bolts.
In the used car biz, aluminum foil tape is applied from the backside, the hole is tapped inwards with a ball pein, and the hole is filled with fiberglass reenforced filler like Kitty Hair or whatever they call it nowadays. �Again, the inside is sorta ugly looking, but you can get the outside to look nice. the same failure ad your JB weld will happen eventually. It may be decades or weeks, depending on how much surface area the KH has to stick to. The really cheeeep used car guys use newspaper slathered with filler to make the patch. I did a Subaru like that once for a buddy and was amazed at how long it lasted....in the NY rust belt too.. We saved a rotting hulk of a car for a couple more years that way.
Smaller holes can be soldered up with regular acid core plumbing solder.or body solder. It is a little more durable that way but then you have the lead safety issues.