Looking at the CREATE TYPE statement in MSDN I don't see where it allows you to name a column constraint like you can with the CREATE TABLE statement.

Your statndards my require default contraints to be named on tables, but it is quite possibile that this won't be possible on table types, nor do I think you would want to. It may cause issues if you use the same table type more than once in a given procedure or function. I also do not see a benefit to trying to naming a default constraint on a multi-statement table valued function.