extendable physics engine/entity interaction

I''m working on a pinball simulation and have an architecture/design question for you all. Please keep in mind that one of the major design goals is to allow end user expansion. So what would be the best way to allow end users to define new entities and their physical behavior without forcing changes to the main physics engine? I''m inclined to have the physics engine be a moderator for a couple of queues (''entity needing attention'' and ''forces to be applied to entities'' queues). The entities define their own behavior either through a simple scripting language or a DLL with a common interface. They would then determine their own reaction to an input given by the physics engine. So before I go charging off implementing this I''d like to get an opinion or two on what pitfalls I''m setting myself up for. --NeverMore