The problem comes in how granular you want to make such a product.
My StatBlockPaster is atomic in nature (not atom-powered, unfortunately) -- a statblock is a single chunk of data that is never altered in and of itself. You COULD offer a program that just has a bunch of input windows (one for Initiative, one for Strength, one for BAB...) but it starts to get VERY VERY complicated VERY VERY quickly, IF you try to make it more granular. The problem is that in trying to CALCULATE values based on other values, you end up with a spectacularly confusing web of relationships between values that is very difficult to manage (PCGen).
I'm not sure what the solution is. Maybe a single-window app with an input field for each statblock section, along with a displayed "Calculated Value" next to those for which it makes sense. So next to "Grapple" it would display the total of size, BAB and Strength modifiers, along with an input box where you could either just enter the provided value, OR, if this creature had some special modifier, alter the value to whatever you wanted. When you changed the creature's Strength, the displayed value would change but not the value in the input box. You could then decide to change the actual value for that field.
You know, I think that wouldn't be rocket science to build. Hm.