how to keep low-res photos handy, while archiving high-res versions?
Depending on how your Aperture library is set up, I think it will do this automatically. 

If you use a referenced library (see the Aperture manual for more information on managed vs referenced), Aperture won't store the hi-res masters in its own library - it will leave the file management up to you. It does, however, keep a low-res preview of the image in the library. So you could store your images on an external HDD, but still have access to the low-resolution previews when the HDD isn't available.

I haven't tried this myself, so I'm not sure how small the previews are, but I'm guessing they'd be good enough for the purpose you're describing.