I've tried it when I have a big (but legal) load with me, and have always been turned away. It's probably always been on United, so maybe they're more vigilant than some other airlines. They seem to have more categories that affect your boarding group number.

There have been times when I've been tempted to carry a cane and go on with the "need a little more time for boarding" boarders. The only reason why I, and probably most "cheaters" want to borard before my assigned time is to assure that there will be overhead storage space, but I find that even when I'm in the last group, that even on a nearly full flight, I've been able to find space overhead near my seat that's within reach or a couple of rows forward so I don't have to wait until the rush clears to go toward the back of the plane to retrieve my stuff.

Maybe I've just been lucky, or maybe they really have it figured out so that statistically everyone who needs overhead space has it. But still, I hang around the line when I see it getting near the end of the group before mine so I can be one of the first in my group to board. There are a lot of cheapskates like me and I try to at least beat out the fair competition. ;(