[CONNECT C++] Dialog Items Access String

An MDL dialog item usually has an access string that connects it to a published global variable.  Here's an example taken from the delivered DialogBoxDemo...

DItem_TextRsc TEXTID_DialogDemoColor =
    {
    NOCMD, LCMD, SYNONYMID_DialogDemoColor, NOHELP, LHELPCMD,
    NOHOOK, NOARG,
    3, "%-ld", "%ld", "0", "253", NOMASK, NOCONCAT,
    TXT_Color,"dlogdemo_colorNumber"
    };

Where dlogdemo_colorNumber is the item's access string.  The item is placed in a dialog box resource like this...

{{X6, Y45, W2, 0},  Text,  TEXTID_DialogDemoColor, ON, 0, "", ""},

In some circumstances, you may want to connect the dialog item to a different published variable.  Rather than define a new item resource, one can override the access string like this...

{{X6, Y45, W2, 0},  Text, TEXTID_DialogDemoColor, ON, 0, "", "access=\"another_global_published_variable\""},

However, I'm having a problem overriding the access string for a ComboBox item...

{ { 13 * XC, GENY(2), 21 * XC, 0}, ComboBox, COMBOBOXID_Purpose, ON, ItemArgInterrogator, TXT_Purpose, "access=\"g_interrogatorVars.purpose\""},

In this case, the ComboBox continues to modify the access string defined in the item resource.  The override has no effect.

Have I done something wrong, or is it impossible to override the access string of a ComboBox?