I looked through the Component Manager documentation, and it seems that it really doesn't want to expose where to find the component. Thankfully, we have the source. Hopefully it isn't too outdated. If these struct definitions are still accurate, you can peek into the RegisteredComponentInstancePtr returned by OpenComponent and then grab its rtiEntry. From there, rtRoutine and/or rtRoutineHandle are probably where you want to look to get the entry point. You probably need to lock the handle in memory somehow so that it doesn't get moved around (Unless that gets done for you somehow?).
https://github.com/elliotnunn/supermario/blob/master/base/SuperMarioProj.1994-02-09/Toolbox/ComponentMgr/PrivateThingManager.h#L79-L112

Some Component Manger related apps:
- "Things!" by Gary Woodcock/QuickTime™ Development Team
- "Fiendishthngs" (has source code) from Apple
- "Reinstaller3" Apple Computer QuickTime 4.0 SDK
- "Thing Detective" from Unthinkable Research
- "Thing Reporter" from Unthinkable Research (free)

I think I was using these when I was reverse engineering the planb (video input of Power Mac 8500, 8600, etc.) video digitizer QuickTime component from Mac OS 9 68K to port to PowerPC and Mac OS X.