I guess that answers my original question then. Since the edge gateways can't connect directly to the database on the network they'll have to show trends from the internal database and then the central gateway will be in charge of logging historical data for long-term use.
edit: are you sure it can't run redundant? The pricing selector allows redundancy to be selected for an edge panel and the status page for edge has a slot for redundancy, along with a redundancy settings page.
Last thing then, I'll need to find a clever way to automatically assign the Storage Provider for tags with history to either "MySQL" at the central or "Edge Historian" at the remote gateways.
Too bad the expression binding on the Storage Provider doesn't allow bindings to tags...