Well, your logic is sound ... but I don't think you should assume that your supply is actually LOW at any time of the day. From your previous posts, it seems more likely that your son is accustomed to a fast flow, and that when your milk flows less fast later in the day, he gets frustrated. That doesn't mean there isn't enough milk -- it means he needs to learn a little patience and apply himself to working for his milk a little.
So -- when one breast is "off shift" during block feeding, yes, it may for a day or three be producing more foremilk when it is finally nursed than it did previously. But after a few days, your body will get the signal that it can ease off on the milk over-production a bit, and then the breast will start producing Just the Right Amount of milk for each feeding. And since your son will be emptying each breast more thoroughly (since he will be learning to settle in and keep nursing even with a slower flow), then he will get the hindmilk along with the foremilk, and his tummy will be just fine that way.
That is the theory. I think you are catching this budding oversupply/foremilk/hindmilk problem before it has a chance to develop into a big problem. Let us know how it goes. (3yo whining at me, sorry I'm rushed to finish this msg now).