Asylum Cycles has had somebody racing their Meuse at the local races this year with a 700x40 Clement MSO and it looked like their was still a decent amount of clearance.
I run 700x35 Marathon Supremes with fenders on my Kona Jake. I'm pretty confident that I could use wider tires (700x38 for sure, maybe 700x40) but I'd have problems getting fenders properly fit over them, particularly at the seat tube.
Those Little Big Bens look nice.