There is a discussion on the Classic Rendezvous discussion list about the use of mountain bike forks at Paris-Roubaix in the 1990s.
I asked Eros Poli to tell me about his experience of riding a RockShox equipped Eddy Merckx in P-R. He replied: "We used the RockShox because it served to damp the vibrations on the pavè, on the smooth road portions we closed it to zero and it became rigid. It was much too heavy and did not give great advantages. They were replaced with forks with more inclination to the front (ed. note: more rake) which damped and tracked better, they also responded better to attacks on the pavè or from a curve."
Photo: Eros Poli, ahead of teammate Henk Vogels, riding a RockShox equipped Eddy Merckx at Paris-Roubaix in 1997