Is there any way in VoiceXML to to trigger an event from a grammar no matter what the context? Like a user saying start over at any given moment during a call?
Take a look at the VoiceXML Link element. You can specify them at the document level or in the root document to make them global and then catch the matching event:
<link event="selection">
<grammar mode="voice" src="startovergrammar.srgs"/>
</link>