cool idea, way better than those clamps IMHO
and still, if you ride a bike like you should, i.e. feeding a lot of input through the pegs, standing up on them when riding over potholes, etc, I would think twice.
Would really hate to have such a peg turn on me, could end in tears.
the least you can do is stand on those pegs and jump on them a bit to see if they move at all.
the one on the left side looks in very close proximity to the chain run , so watch out.
A proper report would be welcome, let us know how it works after you ride on it for a while.
In any case, there are plenty of ways to attach rear footpegs to sohcs without welding anything :-)