Setting mark in minibuffer prompt

[Whitfield Diffie <the.whitfield.diffie@gmail.com>]

    I am trying to write an interactive function that prompts for a
filename, placing point at the end of the prompt and mark somewhere
earlier in the path.  For example, I would like to be prompted:

        ~/notes-directory/2012.05.17/
                                      ^                  ^
                                      |                   |
                                 mark             point

so that if I type a <cr> I get today's notes directory but if I type
<c-x><c-x><c-k><cr>, I get the overall notes directory.

    It is certainly possible to get to this state by hand.  From
today's notes directory, I can type <c-x><c-f> and get the prompt

        Find file: ~/notes-directory/2012.05.17/

if I now move the cursor back to just before 2012, set the mark and
move the cursor to the end of the line, I am in the right state.

    I can also manage to set the arguments to read-file-name in such a
way that the point is somewhere in the middle of the prompt but what I
want is to have the point at the end and the mark in the middle.


    I hope I am writing to the right place.  I have received some
wonderful help on some occasions in the past and silence on others.

                          Many thanks,

                                                Whit