Note in Checkout That Shows Up Geographically Only

Site URL: http://foamnasium.com

I need to show certain content ONLY to a specific geographic area. Is there a way to add a pop-up, or a custom section in check-out, exclusively to California customers? 

Preferably, this would be in checkout, but it could also appear on individual product pages if necessary.

Maybe here's another way to ask... is there to make a Custom Form Field in checkout only available based on Shipping Address? In other words.. a shipping address in California would see a message that a shipping address in New York would not?

No, the checkout page is locked and cannot be modified by users/owners of stores/sites. All your customers see the same thing when they're checking out.

Although you can add forms to product detail and checkout pages through the SS interface. There is no way I know of to limit those forms based on location.

The checkout page is a no go in any case as sruss76 mentions. It's a SS security feature.