Here's a little follow-up.
I first tried the rubber cement as suggested by DWFII, and was pretty impressed with it's ability to lighten by removing oil. Unfortunately, it is hard to really pinpoint where it removes, especially since I spread the problem around with water as shown above. But as you can see below, I was able to splotchily lighten as in the picture below. The lighter parts are where the thicker parts of the rubber cement were. Sadly, it didn't remove the watermarks:
I now wanted to solve the watermark problem, so I took patrickBOOTH's suggestion of vinegar/water. I decided to cover most of that panel of leather with the solution and use paper towels to hold it in place. Interestingly, you can see that some spots simply wouldn't absorb any:
Waterstains are nearly impossible to get rid of.
If this last photo is of the shoe after is has dried thoroughly, I'm afraid it's a done deal. The only remedy I know of is to soak both shoes to the point where the leather is uniformly dark with moisture, and then put a liberal coat of Bick4 on them all over (don't spare the conditioner and don't wipe off, either) and let them dry slowly.
And yes, the leather may end up a shade or two darker but which is worse parts that are darker and so look odd or the whole shoe being darker and no one ...not even yourself after a little while...knowing it was ever a different colour?
PS...A word to the wise--I am hesitant to make that ^ recommendation...I've treated leather with waterstains in this fashion a number of times and had good success, but I've also had failures. Sometimes, depending on the leather and what fat liquors were used, what tanning agents are still resident in the leather, it only makes the stains more entrenched and...well, worse. It's always, the remedy of last resort.
Edited by DWFII - 9/22/14 at 2:04pm