You will probably need to set up a central ePO as a "rollup" server then have your ePO servers at each site rollup to that central server.
Then you can view reports from the central server ePO and view status of all your clients.
I don't have any experience setting up a rollup server so I can't help you there.
hi Aaron, as fuzziest says you can have multiple ePO servers (one for each customer) and then an ePO server for you (the rollup server) that will contain no computers and no policies/tasks but will receive events from any other ePO server through the rollup task so you'll be able to create any report you need.
You can use automated tasks to export reports to any supported format (PDF, CSV, HTML, XML)