Always proportionally scale imagedata to page.

I would always like to have any images scaled proportionally to their  
largest size on the page, and be able to override this for individual  
images. Sort of the reverse of what happens by default.

Would I handle this in a customization of <imagedata/> or  
<process.image/>?

I've tried variants of both with bad results (read: I did something  
wrong and it didn't work at all).

Basically, I want the following attributes defined for all imagedata  
items...

scalefit="1"
width="100%"
contentdepth="100%"

I know this is probably easy, but it's been several years since I  
worked directly with the stylesheets and the cobwebs are thick.

Any help would be greatly appreciated.

-David