Declaring a custom fixity for an infix data constructor should work:
Prelude> data Infix a b = a :@: b; infixl 4 :@:
This is a followup to Trac #2947, which handled fixity declarations in ghci
statements (e.g. let add = (+); infixl 6 add).
Support for declarations (data, type, newtype, class, instance,
deriving, and foreign) was added to GHCi in Trac #4929.