Here's a follow up for anyone who finds this thread in the future. I may have answered my own question but since I'm still learning OP9, there may be another feature yet to be discovered. Optics Pro 9 has a new feature, Export to Application, that might be stretched to consider it a plug-in. You don't install it as a plug-in or even select it as an external editor from Aperture- it seems designed to be a stand alone preprocessor for your RAW files.
If you wish to continue using Aperture for DAM, you'd launch OP9 after importing your RAW files into Aperture. The OP9 Organize tab shows the complete folder structure on your storage drive including all the folders w/in the Aperture container. Folder names are the cryptic number system used by Aperture so navigating could be a challenge in a large library. Anyway, this gives you complete access to all your original files so they can be opened in OP9 using its RAW converter for further processing. Several of OP9 tools such as Prime noise reduction can only be applied to the original RAW file. After processing is complete, you Export to Application & choose Aperture. After you've chosen a file format, OP9 will place the file back into the Aperture container as an untitled project & the file will have "_openwith" at the end of the name. Within Aperture, you can then move the file to any project or folder you wish.
This certainly makes round tripping easier but not as convent as say NIK plugins or what the plug-in Catapult apparently does - put the file back into a stack w/ the parent file.