This thread can be closed, I figured it out. I must've had a syntax error in my XML file initially. At any rate, here is a known working XML file that looks for all MDB's located on the C: drive:
<InventoryClasses>
<InventoryClass name='Cust_AllMDBs' manufacturer='Altiris' description='' version='1.0' platform='Win32' mifClass='Altiris|MyCustomClass|1.0'>
<xml xmlns:s="uuid:BDC6E3F0-6DA3-11d1-A2A3-00AA00C14882" xmlns:dt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882" xmlns:rs="urn:schemas-microsoft-com:rowset" xmlns:z="#RowsetSchema">
<s:Schema id="RowsetSchema">
<s:ElementType name="row" content="eltOnly" rs:updatable="true">
<s:AttributeType name="c0" rs:name="CreationDate" rs:number="1" rs:nullable="true" mifAttrId="1">
<s:datatype dt:type="string" dt:maxLength="255"/>
</s:AttributeType>
<s:AttributeType name="c1" rs:name="FileSize" rs:number="2" rs:nullable="true" mifAttrId="2">
<s:datatype dt:type="string" dt:maxLength="255"/>
</s:AttributeType>
<s:AttributeType name="c2" rs:name="LastModified" rs:number="4" rs:nullable="true" mifAttrId="4">
<s:datatype dt:type="string" dt:maxLength="255"/>
</s:AttributeType>
<s:AttributeType name="c3" rs:name="Name" rs:number="5" rs:nullable="true" mifAttrId="5">
<s:datatype dt:type="string" dt:maxLength="255"/>
</s:AttributeType>
</s:ElementType>
</s:Schema>
<rs:data>
<%foreach wmiobject="o" namespace="root\CIMV2" wql="SELECT * FROM CIM_DataFile where drive='c:' and extension='mdb'" %>
<z:row
c0 ="<%writexml "wmi

.CreationDate"%>"
c1 ="<%writexml "wmi

.FileSize"%>"
c2 ="<%writexml "wmi

.LastModified"%>"
c3 ="<%writexml "wmi

.Name"%>"
/>
<%next%>
</rs:data>
</xml>
</InventoryClass>
</InventoryClasses>