customize CFileDialog for dynamic Files of Type

[sysop HAL9K com]

My boss wants to be able to change Files of Type pulldown dynamically inside a CFildDialog callback. For example, we would start with a list of 5 file types and the last file type is "More...". If the user clicks on "More..." then the Files of Type combo is to be repopulated with 25 file types, for example. Then the last entry is "Less..." and so the user can reduce back to 5 file types. Has anyone seen this done in a commercial product or a demo? I'm no stranger to explorer style hookprocs, written them in (and out of) MFC. I can capture the child control notification, I clear the combobox, repopulate with a new list. However, the next time the user comes along and picks a type, I get a null pointer in COMMDLG32.DLL. Should I give up or is there hope?