Are you talking about the Part Library Search tool used when adding files to the assembly, or the Query tools to search for parts already in the assembly?
In ST8 and prior, it can be done, but it's not entirely obvious. To do it, simply type the property name in the Property field when defining the query:
In this example, I added a "to purchase" property.
In ST9, it will be a little more obvious because we have added an explicit entry called "Custom File" which will add an edit field to define the property name when selected:
Hope this helps,