Not many companies care about offline capabilities...leaving an opportunity space =)
I have not seen coverage check services that provide spatial query interfaces as described, however, there are examples of checking coverage based on location (ie zipcode). For instance, Sprint (if it requests to allow GPS location, select "No" so you are prompted to enter a location): https://coverage.sprint.com/IMPACT.jsp?
The Sprint example seems to use a proprietary servlet implementation rather than OGC...and there are no indications of an offline capability...but Hologram isn't a normal company =). I do think it a decent precedence, though.