I have spoken to people familiar with animal law, and according to them, even dog contracts are hard to uphold in court, because, legally, the animal belongs to the purchaser, regardless of the contract. There are a lot of dog breeders who have been unable to get dogs back even after a breach of contract.
And you seem to misunderstand the "co-ownership" part. I have co-owned 3 different Aussies in my lifetime, and the contract was not what made the co-ownership. The breeder and myself could have both thrown the contract out the window and the dog still would have been co-owned because it was registered to both the breeder and myself through the AKC and ASCA. I could not have done anything with the dog (such as breeding or transferring ownership) without the breeder signing the papers also. However, I could have bred the dog and sold the puppies without AKC or ASCA papers, and have breached the contract if I had and, at that point, the breeder may have fought to get their dog back, but would have never been able to get the puppies back (but may have sued for damages or profits earned off of the litter - depending on stipulations specifically written in the contract).
The contract is not just there to protect the dog, its also there to protect the buyer. Dog contracts typically state that if the dog develops a congenital defect before the age of 2 (or 26 months, or 24 months, or 32 months, etc.) then the breeder will replace the dog (often two options will be given for replacement and some may offer a refund, but this is not common). It also states that if certain conditions are met (ie. the dog achieves a title and health clearances are complete) then the breeder will sign the dog's registration papers over to the new owner making the dog fully and completely theirs. If the purchaser never meets those stipulations or breaches the contract, the breeder can attempt to get the dog back and refuse to sign off on the dog's papers.
The biggest difference here between a dog contract and these mouse "contracts", are that the dog does eventually become the sole property of the purchaser, and the breeder may ask for a puppy back (but the papers will not be signed over until this step in the contract is complete also) or use of the dog as a stud later, but they do not ever place stipulations on the dog or its offspring or what dogs that dog can be bred to AFTER the purchaser is given the papers and the dog is fully and completely theirs.
I don't see how anyone can contract generation upon generation of any animal, as they would have to prove that those animals were theirs, and in order to do so without any doubt, would need to obtain DNA evidence that those animals were off of that line. And that would require the breeder to keep DNA samples of all mice sold on file for DNA comparison if ANY breach of contract occurred.
I have never purchased any mice where I had to sign a contract (in the US). I have had mice where there were stipulations to be met BEFORE the exchange of animals went down, and that was merely becoming a member of a club before they would sell to me, which I have no problem with. And I have made "good-faith" statements to people, such as telling them I would not release their lines for 3 generations, but it was never required. It was done merely as a testament as to my respect for that breeder and her hard work over the last 20+ years. I do not think she really cared one way or the other.
Out of curiosity, why would a contract stipulate that a mouse couldn't be bred to a certain color? What if that breeder purchased that mouse specifically to work on a new variety they were trying to standardize, and that cross was required to make it?
Mouse Breeder, I have heard a lot of different things stipulated, but I have also heard that there a some breeders who use those "contracts" to take back mice for no reason or without any breach of the contract. I also heard, if they suddenly decide they don't like the purchaser one day, they up and try to take their mice back (including descendants) just to be spiteful and hurt the other breeder's breeding program. And (in some of those instances) instead of trying to obtain the animals legally or take the person to court, they threaten, harass, and stalk them at public events. So I've heard. I'm not 100% positive on the truthfulness of these events, but I've heard. In some cases, I even heard tales of mice being given away and then the person they were given to told they were contracted AFTER they got the mice home (several hours away) OR the person claiming there was a verbal contract in place later because no signed contract could be presented.
All I know is, I don't care how nice the mice are, I would never want to purchase animals with such unreasonable stipulations, and would probably just go to a different breeder, even if it does mean a little extra hard work breeding a nicer specimen for show. (I used to do contracts for pet animals 9 years ago, but after learning the hard way they didn't hold up in court, I stopped and just screened adopters more carefully).