While geoserver is serving up WMS layers perfectly for indivdiual items, if I set a base layer for the entire exhibit (even one I've successfully gotten geoserver to serve up for an indivdiual record), I get a blank canvas. Any ideas?
I suspect this is a zoom level issue. What happens if you use a different base layer (e.g. OSM), set the zoom level for the exhibit, then swap out the base layer with your WMS layer.
That is unless you rendered tiles for the entire globe in which case it's likely a projection issue.