Technology for enabling Plug-ins under Tomcat



My client is looking for a framework that will enable her to write and deploy plugins on its system.

Had the runtime environment been JBoss, I would've picked MBeans, but since they are using Tomcat, that is not an option.

At some stage the idea of using OSGi has been brought up. However, transforming the entire system to be OSGi based seem to mean a lot for production site. On the other hand, OSGi features such as classloading domains, restricted module dependency etc. all may be useful in the future. Their question is: What does it mean using OSGi as a part of an application? Does it require changing the entire runtime from grounds up or can it be used only for one,  specific subsystem? can it be used only for loading plugin bundles instead of the entire app? are there any in-avoidable transition sosts (i.e. build, execution scripts, etc.) ?