shovel -
I don't have info in front of me but I think this is a great idea. This way you can have a blackout-proof "safe room" in the summer.
Were you thinking of using a DC off-the-grid Air Conditioner to bypass the requirement of an inverter?
I'm having trouble finding a reasonably priced one. This is NOT an option ($4K!!!) but it does meet your BTU spec :
Sunspot 5000
CarloSW2