I don't think you actually need to do cross-device testing to qualify as "mobile-friendly" in the context of a 48 hour hackathon. You just need to put together

I don't think you actually need to do cross-device testing to qualify as "mobile-friendly" in the context of a 48 hour hackathon. You just need to put together a view @media rules such that your UI is usable on all sizes of browser window and insert a meta tag to indicate to mobile browsers they shouldn't try to emulate higher resolutions. It's impossible to say how hard this would be without discussing a specific web app. But if the benchmark is simply "usable" rather than "elegant", I don't imagine it would be very hard for most web applications.