Supposing all of your types either have a custom generator defined for them, or can be derived by the built-generators, all you really need to do is instantiate a MethodInfo of the roundtrip method for each type you want to test. Then there is an overload Check.Method which you call for each of the types. That will run the standard FsCheck tests as if you would've called the method with the type at compile time.
Something like: (sorry, uncompiled, but should be close)
type Marker = class end
let roundtrip<'a> = //as above
let checkForTypes (ts:seq<Type>) =
let roundtripInfo = typeof<Marker>.DeclaringType.GetMethod("roundtrip")
ts
|> Seq.map (fun t -> roundtripInfo.MakeGenericMethod([|t|]))
|> Seq.iter (fun m -> Check.Method(Config.Quick, m))