In VGC 2012 US Nationals, I actually used a Beat Up Weavile+Cobalion (same great typing as Lucario, a little weaker, but vastly superior bulk and speed) to great effect. The idea was to bluff Fake Out to (hopefully) force the opponent to Protect. With Cobalion's great typing, bulk and speed, I managed to abuse it fairly well. But there's a few flaws though. Both Pokemon are weak to Fire and Fighting moves, so an Infernape would easily ruin your day with Mach Punch/Flare Blitz/Close Combat. The ever-common Hitmontop somewhat limits how often you can use this combo.
Also, if you ever compete in a Top Cut in a VGC tournament, where the matches are done best 2 out of 3, this combo's more of a one-trick pony since people who see the combo the first time can often adequately prepare for it the 2nd time, so you'll have to make sure the other members of your team can pick up the slack when Lucario/Cobalion's not around.
Lucario's an interesting user however since it has higher attack than Cobalion and has Extremespeed, which can heavily dent or destroy a lot of things before they even get a chance to attack, but Lucario seems too frail to survive most attacks in doubles to make a drastic sweep, particularly since it lacks a decent spread move. Cobalion's different in that it has far greater bulk and speed--it can actually abuse it's wonderful typing. unlike Terrakion or Lucario, it can take multiple attacks while striking right back with boosted power. I EV'd my Cobalion to outspeed the standard Zapdos by a few points and to take a few fire and fighting attacks when needed. Cobalion might not have sheer overwhelming power like Terrakion does (it doesn't even have spread moves), but it does have far greater bulk to stick around and ultimately deal more damage in the long run.
Having used Cobalion, I honestly don't think Lucario's viable in a Beat Up combo. Too frail, too slow, no decent spread moves--all of these stop Lucario from being a good Justified user.