How to add metadata to the System.Type?

I've been working on a language, but in terms of .NET integration I've only managed to get primitive types working so far. Last night I had a good idea - instead of trying to figure out what the exact System.Type for tuples, modules and functions should be during typechecking, what I could do instead is get the System.Type of System.Object and add some metadata which corresponds to my own complex language types. What matters is not the exact System.Type I would be adding the metadata to, but that each of the stand-ins for the types in my language are distinct from each other.

The reason I need this is that I am using the functions from the reflection namespace for integration which involves juggling System.Types.

type SpiralType =
    | IntT
    | StringT
    | TupleT of SpiralType list

As an example, what I would like to add to the System.Type would be the instances of the above F# discriminated union type in order to ensure uniqueness.

What would be the best way of doing this?