I don't know a way to turn the toolbar Select Field into one which does partial matching. You can, however, do partial matching by simply typing : and then whatever you want to match into the lister. (Assuming Preferences - Listers - File Display: Use Find field when typing in File Display is turned on.)
I can't see a way to deselect files using the Select Field, nor to create a Deselect Field.
If you want those things, send GPSoftware a request.
Something you can do is create buttons or hotkeys which ask you for a pattern and then select or deselect any partial matches:
(What to do with the XML below.)
Select Partial:
<?xml version="1.0"?>
<button backcol="none" display="both" textcol="none">
<label>Select Partial</label>
<icon1>#selectwild</icon1>
<function type="batch">
<instruction>Select PATTERN "*{dlgstring|Select: Enter Substring to Match}*"</instruction>
</function>
</button>
Deselect Partial:
<?xml version="1.0"?>
<button backcol="none" display="both" textcol="none">
<label>Deselect Partial</label>
<icon1>#selectwild</icon1>
<function type="batch">
<instruction>Select DESELECT PATTERN "*{dlgstring|Select: Enter Substring to Deselect}*"</instruction>
</function>
</button>