Ah, but we do know what they'd do,
because they already vetoed a pretty weak travel/arms embargo after the election mess. I can't think of any reason why they'd change their stance now, and I know that both Russia and China are strong supporters of the principle of "it's just an internal issue, the UN should only authorize force when countries invade each other".
Also the AU wouldn't support it, and the UN seems to like to defer to the AU, who defers to SADC, who defers to South Africa, who defers to Bob.
I'm all for trying stuff through the UN, but right now betting on the UN serves Bob's goals more by buying him extra time until his friends just veto the action away.
Edit: Just to put that further into context, the UN's order of "resolving problems" looks more like:
Travel Sanctions -> Arms Sanctions -> Targeted Economic Sanctions -> Full Economic Sanctions -> ??? -> Invasion
So if they can't get past the starting block, they've still got one huge way to go before they reach the end game.