We attempted this a while back but had to work it around another way. The custom
field type is still our pie in the sky answer, so we will want to address this
one in the future.
Our answer was to create a webpart with the OOTB people picker control. You have
the option of setting the LDAP query as you wish or using the query that is
created from setting the PeopleEditor.SelectionSet. We added a single line of
text on the form and then hid its textbox. When we added the people picker
webpart we appended it to the spot where the single line of text was (jQuery).
When the user chooses a person with the custom people picker we are looking for
the DIV with their userID and adding that to the hidden single line of text.
Our scenario was that we wanted to look up distribution lists from our A.D so we
could send an email to a distribution list from an SPD workflow. That is
explicitly disallowed in the OOTB people picker column for security purposes.
(DL's don't have a security token.) In SharePoint the people picker is used
(almost exclusively) for security permissions on lists, groups, sites, etc.