How about filling through the liquid out tube (fill from the bottom) then hook up a tube to the gas in post (to release pressure as it fills). Put the tube in a glass and stop transferring when beer comes out.
This is essentially how I fill kegs at the brewery. The main difference is it is through sanke fittings and the beer is carbonated so I am looking for foam coming out. It is also chilled, so I can feel the side of the keg and feel the liquid level.
I have done it with a scale as well, but don't prefer that method.
By the way, I have a thing similar to the keg gauge you saw, it is for propane tanks. It works the same way and would work on a keg, although the magnet probably won't stick. Even if it did though, I think they are too annoying to use and would not buy another one.http://www.amazon.com/GrillPro-00030-Magnetic-Level-Indicator/dp/B000FHZ33C/