Unicode -> string? Invalid code point! // Adding a menu item with unicode characters

[David Reitter]

How can I specify strings for a menu item that have unicode-range 
characters in them?
To be concrete, I'd like to specify the 'command sign' character (to 
show a keyboard shortcut with the command key). The unicode ID of this 
character is 0x2318, and I assume I'll have to convert this to the 
internal multibyte representation first. But when I do

(make-char 'mule-unicode-0100-24ff 8504)

I just get an 'invalid code point' error.
Is that the correct charset for the job?

When I have my character, will something like (make-string 1 (make-char 
...)) produce a good multibyte string, and can I pass that to my 
easy-menu or define-key as a text for the menu item?

Many thanks
- D