Paris-Brest-Paris Randonneur has had a category for MTBs in some previous editions (one bloke rode PBP and the World Crosscountry Championships on the same bike) but I've not noticed that option for a while now.
I know several people who use converted MTBs for long road rides. The 24 hr TT record was broken by Andy Wilkinson (541 miles) last month. He uses the same custom frame for record TTs, touring with panniers and off-road riding (uses V-brakes and flat bars, swaps wheels and suchlike).