"It seems like the low cost of glass marbles and the ease of cleaning and sanitizing them is worth the piece of mind that some people have with airspace in secondaries. Autolysis. Oxidation."
No matter how low the cost, it's too much to "fix" a problem that doesn't generally exist with beer. Unless you missed your marks in terms of volume, 5 gallons of beer in a 5 gallon carboy means headspace isn't really a problem. The beer releases enough CO2 to drive most of that oxygen off.
Autolysis isn't a problem at all in a secondary - an not in a primary either unless you are talking about a year of aging or something.