Thought of the same thing
but for a different reason.
Some one correct me ,, if I am wrong,,,,
but by adding foam,, in reality you are just adding weight. Unless the yak capsizes and fills with water. The reason I say this is ,, the yak is floating just fine with out foam. And by adding the foam you are just filling air space with a substance that weighs more than air. Of course you will be adding rigidity to the yak and this is what I had in mind.
My yak has a dent in the bottom that tends to pull me to the right. Not bad but noticable.
If you do decided to use foam,,, BE SURE it is closed cell otherwise it will absorb water and get VERY heavy.
good luck lou