run/debug configurations with classpath different than project?

I need to create a run/debug config where I can specify the classpath manually.

Why? Simple. We are developing an Eclipse-plugin-based application (RCP). It is launched with a single JAR in the classpath (startup.jar) and has it's own classloader for loading the various plug-ins.

I can add the startup.jar to the classpath and successfully run the project. However, running in this manner is NOT the way the application will run when it is deployed - which means there are a lot of configuration errors that we could not catch (or debug) using IDEA.

Note: Our company (and many others) has committed to Eclipse as a development platform (not IDE), but all of our developers use IDEA and would prefer to keep it this way. If we cannot run the app in the correct configuration, then many of the developers will be forced to abandon IDEA for Eclipse. Ugh.