Is there a metric of the overall excitation of the brain or nervous system?
MEGs using SQUIDs can detect magnetic flux density measured in teslas. Away from sleep studies and epileptic brain activity, this should be a useful indicator of relative brain arousal. Readings from two different people on this scale should be comparable  based on the orders of magnitude for the unit measurement. As for comparative measurements of brain arousal, I don't think Actigraphy would be useful, nor any other method of measuring somatic motion. These can certainly determine weather a person is sleeping, sedentary, or exercising, but discriminating between different sedentary behaviors would be difficult and determining relative brain activity for those behaviors would be reliant on more traditional methods (EEG,MEG, etc). MRI methods might be useful, as the total change in blood oxygen volume from a base state or inactive state can be distinguished from a more active state.