Thanks for working on this, Brad! I imagine it's difficult--hop utilization is already sort of a subjective thing, especially when there's not really any good way for homebrewers to measure it.
Basically, the way I understand it, is that you're looking for a way connect wort gravity to hop utilization. Presumably, the program already does this--right? I mean, how else would you get to the IBUs that are generated? I don't know much, if anything, about brewing chemistry, and I know less about the mathematics, but would it be safe to assume that with a late extract addition, up until the actually addition itself, you're boiling a 1.000 specific gravity wort?
Let's say you're doing a 60 minute boil, total. The extract will be added to reach a gravity of 1.040 with 15 minutes remaining in the boil. To keep things simple, let's just say there's a single hop addition, at 60 minutes, with a pellet hop of 10% AA. I may be oversimplifying things here, but--could the hop utilization be calculated with a 45 minute boil of a 1.000 gravity wort, and then a 15 minute boil of 1.040 gravity wort? I realize that none of the calculations here are direct--there's curves and diminishing returns and all the rest of it. But then, I don't think we need 12-decimal point accuracy here... a ballpark figure might be suitable.
Of course, my batches make the situation more difficult, because I do partial mashes and add whatever's extracted from that to the boil first, for the full time, and then add extract later. However, I think the same principal would apply. Of course, then we've potential carmelization of the wort that could factor in, along with a buzillion other things. Hmm... I'm beginning to see the difficulty here.
Well, I don't think I added anything of use here
Best of luck with this--I wish I was more of a help!