Unless your name is Tom Stuker (and you have 10 million UA BIS miles ...)

http://www.airlinereporter.com/2011/...million-miles/

for him, they'll hold a full-747. Or they'll just roll out an empty plane if need be. Otherwise ....

Try to bear in mind that holding up to avoid your mis-connect means they risk a series of additional mis-connects down the line. As others note, having your GA call ahead is the only realistic way for this to happen ... if there were, say, 12 on your flight connecting to, say, Sydney, UA would most likely hold the flight without your having to call (12 hotel nights, meals and rebooking to the next Sydney flight = serious money).