I can't see them winning. It would set a precedent where anyone who designed anything for a corporation would have to get reimbursed for their designs. Just think about the car companies that would have to pay out to every designer who worked on every car they ever built. When you work for a company, you are agreeing to work under those terms. I think if it made it to the Supreme Court, they would say the same thing, that the person in question agreed to those terms. Now, they could rule that going forward that was unlawful and in the future would have to pay people for their designs.
That's not how copyright law works, though. The thing about copyright law is that it's controlled partially by principles of equity, but also by very specific laws with very specific doctrines interpreting them. The designs on cars are a tricky beast because that tends to be a mix of patent and trademark law, each of which have their own highly nuanced sets of laws and doctrines controlling them. You can't really analogize from one to the other, except in the most basic sense, and that ends up giving way to the legal nuances when you start digging into things.
That said, "works for hire" -- if I remember my copyright law -- involves the worker assigning all rights in the underlying work over to the company for which they're working. That means all rights to produce what are known as "derivative works" which includes works in a different medium (e.g., film, electronic versions of their printed works, sequels/prequels/offshoots based on the story, etc.). This is different from a license agreement, which is where a creator gives another entity a right to use the work in some way that would otherwise be reserved for the creator.
A lot of you are missing the point that these artists and writes are seeking additional compensation because their work is being used beyond the medium for which it originally was created and for which they were paid. They invented these characters and were paid for work that appeared in print. Once the characters are transferred to other mediums--film--the writers/artists should be compensated for use of their creations, but they're not.
About a decade ago, Kurt Vonnegut and other novelists brought suit against the publishing industry for releasing their work as ebooks. Publishers asserted that the authors were paid for the work and weren't entitled to additional compensation, while the authors maintained that their contracts were for paper books and that if the work was being released in a new medium then they, as the creators, maintained ownership requiring financial compensation for their use. The judge agreed with the authors, so the precedent has been set.
While Kirby (and no doubt others waiting to see how this plays out) was paid "for hire" that was for work appearing in print. Also keep in mind that the films based on characters Kirby created have made billions and that even a compensation of 1% would mean millions to Kirby's estate. Surely Marvel and Hollywood can spare 1%.
The Vonnegut story would be highly dependent upon the documentation signed between the parties. If he
assigned his rights, or his books were treated as a "work for hire" with the publishing company, then the publishing company could tell him to stuff it because they own the work outright. If he merely
licensed his rights, then the publisher can only exercise those rights that were licensed to it for the duration of the license. If, for example, the publisher only had the right to produce copies
in print, then it couldn't say "We're making a movie of your book, and you can just suck it." They don't have that right, unless Vonnegut signed it over to them. Likewise if he licensed the right to print the book only for 15 years, once 15 years is up, they can't say "So, we're gonna keep printing the book, and you can just suck it." Again, they don't have that right -- it reverts back to Vonnegut at the end of the license. The question in that case, I'd bet, is more about whether an ebook counts as "print" or is a separate form of electronic media, which would require a separate license.
With the Kirby estate, the question probably depends on the terms of whatever agreements were in place between Kirby and Marvel. If Kirby was just an employee or contractor of Marvel, and his works were "works for hire," then as much as it might be
nice to give him credit and money, Marvel isn't legally
obligated to do so, because Kirby never owned the stuff he made in the first place. I'm all for Kirby getting recognition for his work, but that doesn't mean his estate is entitled to a dime from Marvel. If Stan Lee negotiated better contracts for himself, then bully for Stan, but that doesn't have jack to do with Kirby. (sorry, too good to pass up)