The trouble of having a "frozen snapshot" is that the snapshot would have to be identical to the actual release for it to be useful in updating mods. You can't just say "Oh, yeah, we'll be changing this function to work in this way, deprecating these so that we can consolidate them into these others which are more efficient processor-wise" and then modders work from that. The way one individual interprets these changes may be vastly different from how they're actually being implemented, without having the actual finished product in front of them.
That said, as I understand such things, most commercial programming ventures rely on a design process that includes a finalized sort of "storyboard" chart (I can't recall the exact term) rather than making adjustments on the fly. So, assuming they use that sort of process, they could release that; such things are designed specifically to be interpreted in only one way, because they're most often used by large commercial ventures where the design and program flow is independent of the people who are actually writing the code. Smaller projects tend to favor greater overlap between design and coding. In any case, even presuming that such a thing wouldn't be a legally horrible idea, child mods would still have to wait for the parent mod to be updated to see how they're going to update themselves.