I wouldn't recommend that. The way you should store kegs, per John Palmer, is dried, upside down, lid loose, and the posts lose. If you store them with StarSan you are going to be leaving them with an acid for however long. Plus, you will effectively be creating an oxygen depleted state however high you're filling it with this acid.
If you're worried about dust and stuff, when you're ready to use it give it a quick rinse, StarSan, use.
I'm not saying that if you store it the way you're saying it will be ruined in a month, not at all, but I just think it will be negative for the stainless steel.
Edit: this is from another post:
"This is what John Palmer says: Phosphoric in 300 series stainless steel is generally okay, according the ASM Metals Handbook, Vol. 13 - Corrosion.
However, it does go on to say that any impurities, particularly chloride and sulfate, will accelerate pitting and crevice corrosion.
So, I will bet that those of you who have had pitting problems at the water line, or other etching, have high chlorides or sulfate in your water. I never recommend storing anything wet. Don't care whether it's water, or Starsan or PBW or Onestep or vinegar. When you store metal items wet, you allow all the time necessary for oxygen deprivation of a local area to occur and then you have an activity difference and galvanic corrosion can start.
Pitting at the water line is a prime example. Pitting at a dent or ding is another. Weldments are particularly vulnerable because the metal chemistry/structure is generally different.
You know how I store my kegs? Upside down and open. Sure I get a spider or two in there once in a while, but they are easily taken care of, and I just fill the keg with starsan and let it sanitize my racking cane and siphon. Remember, I am just as FOS as the rest of you, but I hope this helps.
Cheers,
John"
