I've never tried it. I would think it would take away from the softness/form fitting flexibility of the foam against your contents. I would assume it would be wise also to be cautious of other chemicals in the spray you intend to use. Some chemicals may not play nice with the foam compound causing possible damage or chewing it away. Just a thought.
Also, I would think any moisture on your weapon & gear would stay on it because the plastic dip/spray would form a barrier, keep it from wicking away into the foam.