they do this with trials motorcycles all the time, cromoly frames filled with helium, not a huge weight savings but every ounce counts.
Indeed it does, but it takes a cubic foot of air to count to one. On a motorcycle chassis, I'd bet the fitting for filling the frame with helium weighs more than the air that the helium displaces.
This is a pretty common technique (at least at the World Championship level) for engine mounts for competition aircraft, and it does save weight, because you don't have to build with as high a safety margin as you would if you had to wait for your engine to fall off to know you had a crack. The weight savings comes from the confidence inspired by knowing you'll have some notice, and if that pressure gauge drops to zero then you're headed off to the nearest safe place to land.
In practice, we use air, and use tubeless tire fittings as the frame fillers ports. You can find compressed air at any gas station (pretty much) or from a cheapo plug-in-the-cigar-lighter 12v compressor, but where do you find helium in (for example) Poznan, Poland?