Here’s a titanium Eddy Merckx (built by Litespeed) converted to 650B tires and mounting up a Rock Shox Paris-Roubaix fork. Owning such a Rock Shox fork myself, I strongly suspect that the 650Bx38mm tyres would be more of a benefit than 25mm of suspension if taking over the whole P-R route, but over the worst of the pave the tyres and fork together would probably both be welcome. This is bike was assembled as a homage of the suspension road bikes used by professional teams at Paris-Roubaix. In fact, the race was won by a Merckx frame with a Rock Shox fork in 1994, but the frame was steel and none of the bikes in the modern era have used 650B tyres.
Suspension forks, seatposts, and even full-suspension road frames were a flash in the pan at Paris-Roubaix during the early 1990s. Heavily influenced by the nascent mountain bike industry, road suspension had three straight victories in the spring classic but also several spectacular failures. Later on a Belgian team, whose frame supplier forbid suspension, had the race in a stranglehold for years. Other than an elastomer bumper/pivotless suspension seatstay (not to be confused with elastomeric vibration dampers such as Specialized’s “Zerts”) a few years ago, suspension designs utterly disappeared from Paris-Roubaix in the early 00s.
For a history of suspension bikes at Paris-Roubaix and the professional riders using them, check out this post from last year.