I develop an Eclipse RCP application which makes heavy use of OSGi bundles which provide services for later use. The use case requires the bundles to register their services
MANIFEST.MF has this:
Bundle-ActivationPolicy: lazy
This could be also useful:
http://wiki.eclipse.org/Lazy_Start_Bundles#Should_I_change_to_the_new_Bundle-ActivationPolicy_Header.3F
However I would say I would try relying on OSGI activation as much as possible. If there is no other way of solving your problem, then the previous link might help.