No, they've never used EVA repair robots. Part of the Jefferies concept was to have as many systems as possible accessible internally so that a crew could avoid EVA. He felt that would make more sense construction-wise and would be more futuristic. It wasn't until VOY and ENT that we saw EVAs somewhat regularly.
Of course, I've thought robotic EVA repair makes sense. I remember Seaquest had maintenance bots controlled by virtual reality controls inside. That makes sense to me for Trek. Have a VR suite where your maintenance guy controls the bot without danger to himself.
While I've always agreed with and liked Jefferies concept of having the systems accessible from the inside, it seems to be quite the statement on your part to conlusively say that no robot has ever been used in an EVA situation (with the exception of on screen of course- then yes, you're right, we've never seen one used in that situation that I can recall).
We've seen automated systems on more then on occassion. The SOL System drones in TNG. The experimental repair robots in TNG that became sentient <sigh>. Data...
So based on that alone, it's plausible that robots have been used for external ship repairs.