I don't believe it's possible to only use VBScript to add events for onaveragestop (or any event). The APL or XML structure of for your feature layer within an AXF file typically gives the user access to the events' code areas where code can be inserted.
If you wanted to avoid customizing forms or having whatever you want to do tied to a specific layer you could still access these events using an Applet APL with SystemObjects/GPS added, as opposed to an APL that included forms.
Is this what you're asking?
