I arrive at Boston after a long trip and pick up my checked luggage. I then want to visit the Admirals Club, for which I'm allowed past security with no ticket or boarding pass. However, I now (legitimately) have two big suitcases. How am I supposed to get them through a baggage sizer - even though my reason for wanting to do so does not involve wanting to board an aircraft?

Good riddance to the darn things. They are not a security issue. If airlines want to enforce baggage size restrictions (which in itself is problematical when airlines having different size limits share a concourse - whose limits do they make the template for?), let them do it. Keep security screening for security.