Stay patient - just trying to help you here, Andrew.
If it were me trying to better identify this traffic, I would increase the sampling and port monitoring. With sFlow, the load is not invasive - and I use Procurves with all ports monitored in some instances with no hiccups.
Based on your descriptions of your remote sites, I would say your chance of excessive/measurable broadcast traffic is low, short of a malfunctioning NIC/eccentric printer.
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