You can't upgrade using the serial port on the dock because the empeg only listens for upgrades when it's on AC power, and it senses AC power by the physical plug being inserted. Mark does some tomfoolery with the docking connector and makes the empeg pretend to be on AC power, but he can't do this before hijack is loaded, which is when upgrading happens.
It is possible to put a dummy jack in the dock so that it knows it's on AC power, as Tony recently described doing, but this has the downside that you're causing rather rough wear on the AC Shensor Shwitch every time you put it in the dock.
Another option that I don't think anyone's used is to get the barrel connector from radio shack and just put it in your empeg befor putting it in the dock when you want to upgrade. Or just use the ethernet upgrader which is faster anyways.
Matthew