I certainly could laser a small hole in the stainless steel tank.
As for soft solder, there is a product, Stay-Brite “silver solder
kit” that comes with a flux that is supposed to work well on
stainless steel. I use this product to do repairs on pewter type
metals with the laser, and not generally do any soft soldering with
it, but as I recall it was effective on stainless the last time I
did anything like that.
Laser welding the hole in stainless would be a more permanent repair