Looking back through Q. Pheevr's archives, I rediscovered his post about the naming of He Who Must Not Be Named. I think Q might not be entirely right about this.
While I'm not disputing the assertion that He Who Must Not Be Named is, in the Harry Potter books, actually a name, I don't think the invariant case is the reason.
When I check my own judgements, I find that I'm perfectly happy with, for example,
You'll have to get those numbers from she who approves the budgeteven if "she who approves the budget" is a one-time innovation and not a common nickname for the accountant. On the other hand, I can't think of any context where I could use
*her who approves the budgetas a constituent NP.
In fact, it seems to me that nominative is the only acceptable case in definite descriptions that start with pronouns. Maybe this is required for the pronoun to govern1 the rest of the NP?
Further, if the invariant case were simply a result of the description being a name then we would expect it to be possible to have a name/description with a different invariant case, for example
*Him who was thrown down a well came home todaybut I don't think you can. There might be some sem/prag issues interfering here though. When you have Someone Who Cannot Be Named, usually it's because they hold some sort of power over the people who Cannot Name them, and such people tend to find themselves (or at least the NPs that refer to them) in agent positions in sentences.
So what do y'all think? Do people's judgements differ from mine?
1. Apologies if this is not the word I'm looking for. It's been four years since I've done any syntax, so I'm counting on the linguists in my readership to just kind of know what I mean. (back)