I've written a script that will transform any headline into one that follows these rules:
The script isn't perfect: it won't keep acronyms upcapped (e.g., NSA will turn into Nsa). But it will lowercase the following words:
a, an, and, at, but, by, for, in, nor, of, off, on, or, out, so, the, to, up, yet
View the outline here. If you copy the headline to your clipboard, you can paste it into your menubar.
I've rewritten the script to compare each word in the headline to an array of words to keep lowercase using a for loop within a for loop. You can view that version here.