Hi Brian,
I think what you want to do will work, but I'm not entirely sure. The easiest way to think about this is to think about how GeoEvent and ArcGIS Server interact. To put it bluntly, ArcGIS Server has no idea this thing called GeoEvent Processor/Extension/Server exists. To ArcGIS Server, GeoEvent is just another client that makes requests, for features via Map Service or Feature Service endpoints, for example. So when GeoEvent polls for features, or imports geofences, or adds/updates features to a feature service, it's making standard ArcGIS Server request for features, or edits, at the web service tier. This is as opposed to some back-door or under-the-hood communication between the two products that Esri may have built into the two ArcGIS products.
So to kinda answer your question, if you can publish a feature service where the related table's attributes are exposed, and editable, via the ArcGIS Server REST endpoint, then GeoEvent would in theory be able to update those attribute values. I rarely work with related tables, so I'm not swearing by this, but all you really need for add/update a feature is the proper REST capabilities on the feature service, and the right fields exposed.
Hope this helps.
Mark
