The way Russia has been about this I do not think that it is beyond the realms of possibility that they would go so far as to stage an attack on the convoy and make it look like the Ukraine army did it. Then they have an excuse for a military escort and whammo - Russian military in the Ukraine.
I had a similar thought, I doubt they'll do a false flag attack but I can see them using any interference on the convoy by the Ukrainian military as an excuse to send in the 45,000 troops they have massed on the border, and if I were the Ukrainian military fighting in eastern Ukraine I'd be very unhappy about a suspicious convoy from Russia being delivered to the rebels in Luhansk and Dontesk. It's the same tactics they used on Georgia in 2008 - provoke until the enemy responds then do what you like.