Oh, I figured it out! I was trying to do it in a too complicated fashion. The "Restricted" function was actually needed for my other fields, but with Boolean it's this simple, and it even creates that nice little checkbox into the Web Form:
CreateDatabin["Interpretation" -> { "foobar" -> "Boolean"}]["WebForm"]
Now I'm only wondering if there's a way to set it to true by default... Or to set a default value for any other fields for that matter.