Some type systems do support equi-recursive types without the type constructor, e.g. Whiley (http://whiley.org/2013/04/21/iso-recursive-versus-equi-recur...). Maybe there you could implement Y without a type signature and have the recursive type inferred.
The main problem is speed. Using the Y combinator is going to mess up whatever code flow analysis the compiler has, unless it's using some cutting edge optimization research that I haven't been able to find.