Is this possible with an action?

I have a whole buttload of files that I need to resize, but they aren't all going to be resized by the same increment, so a simple batch resize won't work. I'm wondering if it's possible with a more complicated action or script.

Basically, the files are a bunch of musical notation, and they need to be resized so that the height of each staff is 5 millimeters. But each figure may have just one staff or several, and the different notes going above and below each staff make each figure a different size.

So far, I've just made another file that is exactly 5 millimeters in height, and I've been dragging that into each of these figures and pulling guides to its edges, and then eyeballing with the scale tool. Can anyone tell just by this information if what I need is even possible? Or am I stuck doing these figures (thousands of them) individually?

You might select layer transparency (if figures are on a separate layer) or luminosity and crop to selection