I don't want to be the rebel here, but it's entirely plausible to run onto the court if the C's win the championship. There aren't enough security guards in a single arena to stop 18,000 fans from rushing the court. I have firsthand experience with this. I witnessed a certain favorite college team win a national championship in mens' volleyball not all that long ago. There were security guards that tried to discourage and prevent people from rushing the court, but one security guard can't stop more than 1 or 2 people at a time. Unless the entire Boston police force is guarding the court (unlikely), a couple hundred brave souls could easily overwhelm the Garden security people and open the floodgates for everyone. And it can absolutely be done in a civil manner and without posing great injury risk.
If Boston wins the title and they do it in the Garden, I would love to see the fans rush the court - it would only be fitting. The problem I see isn't so much the security guards - the problem is that the fans in the lower levels have to start the rush and I'm not confident that the suit yuppies in the lower levels have that kind of enthusiasm. Maybe I'm wrong. I haven't actually been to a game in the Garden yet (I have tickets to game 2 - I'm flying out from Oregon), so maybe the Boston crowd is different. I sure hope so. But if it's like other NBA arenas, I don't know if anyone will pull off a court rush.