After many more hours of messing around, i think i solved the problem.
Exporting the product creates a
configuration\org.eclipse.equinox.simpleconfigurator\bundles.info entry:
org.eclipse.equinox.p2.reconciler.dropins,1.0.100.v20090520-1905,plugins/org.eclipse.equinox.p2.reconciler.dropins_1.0.100.v20090520-1905.jar,4,false
Changing the autostart parameter before packaging from false to true will
allow discovery of new dropins and plugins without the need for the
config.ini.