I just pushed a partial fix to the long-standing problem of not being
able to use user-set bookmark titles in the read-url completion.  There
is however a small catch.  You can only complete on either history or
bookmarks, but not both at the same time.  Therefore, put into your rc
one of the following:

   url_completion_use_bookmarks = false;
   url_completion_use_history = true;

  *or*

   url_completion_use_bookmarks = true;
   url_completion_use_history = false;

  We hope that this limitation will be short-lived, because the Mozilla
source includes a note about the need for a particular flag called
QUERY_TYPE_UNIFIED to be implemented.  This is the flag that conkeror
depends upon in order to do completion on history and bookmarks at the
same time.

  Here is a toggle command and binding you can put in your rc:

function url_completion_toggle (I) {
    if (url_completion_use_bookmarks) {
        url_completion_use_bookmarks = false;
        url_completion_use_history = true;
    } else {
        url_completion_use_bookmarks = true;
        url_completion_use_history = false;
    }
}
interactive("url-completion-toggle",
            "toggle between bookmark and history completion",
            url_completion_toggle);
define_key(content_buffer_normal_keymap, "C-c t", "url-completion-toggle");