It'd interesting to game the scenario out. What would happen if the US (or another NATO or EU nation) took out the convoy?
Well, it would take a lot of pressure off the Ukrainians, potentially freeing up many of their forces to attack the Russians in other areas. It'd be a devastating logistical blow ("Generals talk logistics") which would likely cripple the entire offensive in the North.
How might Putin respond?
1) Well, he could go tit-for-tat, and start shooting SAMs at the various ISR and refuelling aircraft that are circling just west of Ukraine, or he could lob some missiles at NATO military bases near Ukraine, but that's an escalation. Attacking a defenseless aircraft in the airspace over a NATO nation would provide all the excuse necessary for NATO to open the floodgates.
2) Ramp up the attacks on civilians in Ukraine? That wouldn't accomplish much.
3) Perhaps he could declare war on NATO? Well, see #1 above.
4) Start using nukes? See #1.
This all assumes, of course, that Putin can pin this on NATO or the EU. Given the Russian military's performance to date, I imagine the only argument they'd have is "well, Ukraine couldn't have pulled it off."
I don't know which way this war will go in the short term, given the status quo. Without the international sanctions, I think Ukraine would lose--Russia could just keep feeding men and materiel into the war, and attrit them out. With the sanctions in place, it's a matter of timing--can the Ukrainians hold out long enough that the sanctions will hurt enough that Putin is forced to withdraw?
Personally, I would love to see a volley of Tomahawk cruise missiles loaded with cluster munitions run right up the length of that convoy. Or maybe, instead of cluster bombs, drop leaflets: "This could have been a tiny bomb......turn around and go home before the next wave of missiles arrives."