Not in the show that I can remember, but he does it with Kermit in Muppet Classic Theatre during the Elves and the Shoemaker act.
I also agree, Matt would be very good as a backup for Bunsen.
Matt soundsso very close with Robin, but he might have to remember to keep a more kiddish squeakier inflection when performing him and if he can manage it, make his voice a bit higher.
Also, I feel as if there should've been more interaction between Kermit and Robin, more looking at each other, maybe Kermit putting a hand on Robin's shoulder when they all sing together. Robin seems so static compared to Kermit, who was moving his head while he sang instead of just staring at the camera. Even just Robin glancing at Kermit while singing would've made it all the more convincing and natural.
I also agree that Robin needs a lighter shade of green, he looks a bit sunburnt.
But yes, so I don't sound like a whiny wuss, I love that little Robin hasn't been forgotten and them singing together just makes my heart melt. I really hope we get to see more of him from now on.