>   #type 'a mytype = Mk of #myclass as 'a;;
>   Unbound row variable in #myclass

In a type definition, type parameters *must* be free variables. But, here,
you try to bind the type parameter 'a to #myclass. So, this definition
fails. Actually, the compiler sees that #myclass contains a row variable
that obviously cannot be a type parameter, and hence rejects this phrase.

You can remove the constraint and just write
    #type 'a mytype = Mk of 'a;;
I don't think omitting the type constraint is a problem in practice.

However, I plan to add constrained type definitions to the language in the
future. Then you will probably be able to write your type definition as
    #type (#myclass as 'a) mytype = Mk of 'a;;

        Jerome