Bruno and canuckInOR (wait, wasn't that LA?) better described my vague idea. So, the answer is:

Fact - Photoshop applies changes to the current version of the layer, not some cached copy of the original.

So, I'll just have to continue the two step process of roughly laying out my compositions, then resizing source images only once for the final production.

I mean, I only use Photoshop (7) for glorified cut and paste. I really hoped it would make that seemingly simple job a little easier.