The biggest problem that you have to take into consideration is that players are extremely mobile compared to robots. Normal mvm works because the bots have to do a slow linear push into your territory, a push that has the bomb as its epicenter. It's easy for players to have a broad overview of what's going on because of it.
If you want to reverse the roles, several unwanted things suddenly become very real problems. Player will, with their mobility and superior intelligence, be able to push into territory with extreme ease regardless of the resistance they meet. If you went with a payload cart, which would indeed make sure there was a easily recognizable progression through the mission, you'd have to limit the players in their movement. Something that could have a negative impact on how attractive the mission is to players.
Normally, players have to defend territory, which means that they'll lose ground if they retreat to recover. To reverse the roles, the robots would have to reclaim territory if the players pull out of battle, or the players will be able to retreat without penalty all they want. This could probably be partially countered by giving robots certain bonuses and healing when they live long or survive a lot of damage, etc. I am not sure how you'd accomplish this tho. The bots would also need a lot of entity stuff about to determine which areas they should try to defend, since bots have no concept of strategy. In normal mvm, it's easy for them since they have to follow a path while killing anything that opposes them. In a reversed situation, however, you would need a very intricately ai guiding system to make all the tactical choices. I am not certain how you'd do that either (altho i'm relatively certain this one is actually possible).
I think the best way to make an offense-based mvm map would be to have it revolve around some sort of boss entity. A mvm version of
this map basically.