What you want to do can easily be achieved, by turning your library into a referenced library.
Relocate most of the original master files to an external volume, only keep the originals from the projects you are currently working on, on your MBP. Set the Aperture preferences for previews in a way, that high quality previews are rendered for all your image versions - set the preview pixel size at least to the dimensions of your MBP display. The high quality preview jpegs will be available to browse the images; you do not need to create separate jpeg versions.
This way, you will be able to browse and tag all your images using the jpeg-previews; you can even use them in the Media browser in other applications, when your external drive is not connected. Only if you want to export or to edit your images you will need to connect your external volume. That is why I'd recommend only to relocate the originals of older projects and to keep a working set of current projects on your MBP.
For more information see: Aperture 3 User Manual: Working with Referenced Images