To crunchy worms as CRBs are to GMO cows, if you will.

The item would be fed to the dam only and would roll per cub. When triggered, the item guarantees the cub's base will come from the same genetic category as hers.
The relative rate of passage of each base will remain the same as in any other circumstance and she will not be able to throw bases requiring a heritable base she does not have. nor will she be able to produce the new combination bases, even if they fall in the same genetic category as hers.

Thus: If used on a palomino lioness, if triggered, the cub will have a 25% chance of being either copper, palomino, sorrel, or tawny, as all common bases are (as far as I know!) equally weighted against one another. If used on a sapela girl, and triggered, the cub will have a 50% chance of being either cinnabar or sapela, as (again, as far as I know) all Oasis applicable and regular breed-only special bases are equally weighted against one another. If used on a skyward, and triggered, the cub would have a smaller chance of being skyward and a larger chance of being chartreux, and no chance of being glacial or opal. I believe the over-all pass rate of most heritable breed-only specials is somewhere around 5%, but I don't know if anyone has done the math to figure out their roll rate relative to their fails. If used on a prune, only other prunes would be possible, and not unholy or madagascar cubs, but if used on an unholy, both unholy and prune cubs could result, with the bulk being prune, and a madagascar could only produce prunes using this item.

I imagine the item having a success rate of 50% per cub. This seems a little high at first glance, but, look at it this way. Crossing two special based lions provides a 30% rate of producing a special and, save for medium bases, all other aspects of a base's genetics can be locked into by breeding within the same color, shade, and and gradient. Finding a special based stud is incredibly easy and often not even expensive. What's harder is getting a special based lioness, especially one with a heritable breed-only base. As this item can only be consumed by dams, like the crunchy worm, it serves only to boost their value as something besides sex-change fodder.
Since its obvious use is in upping the odds of producing those heritable breed-only bases, most pairings will be set up to already provide a 30% shot at producing mom's base genetics anyway in case the item fails. Thus, it's basically a 20% boost in odds per cub of rolling in the desired category, and nothing more. Unlike the worm, it does not guarantee an exact pass of base nor does it improve the passing rate of anything else. On the other hand, if you want mom's base but dad's markings, eyes, or mane, then this item is the better booster for your project, since the worm doesn't let dad contribute.

It would be a slightly higher boost in odds of obtaining those difficult medium special bases than of other special bases, bringing the odds-of-category-pass up to 50% as opposed to the lesser chance normally afforded due to the impossibility of 100% medium shade litters.
It would also come in handy for those breedings where the stud with the mane and markings you want has a base in a category you abhor. We've all been there.

I see no reason why the item could not stack with the worm. Much as CRBs stack with GMO cows to up the odds of muties per litter. these would stack to up the odds of base passage. However, since worms roll only once per litter, the worm should roll first. If triggered it would pass the clone to a cub, and then this item would roll for the remaining cubs. After that, natural odds would roll for any cubs in which neither item triggered. If both items were used on a lioness and she had a one cub litter, if the worm triggered, the proposed item's effect would be duplicitous and thus may as well have not existed. Go buy a GoP, or something.

I think it might be nice to roll out an item of this nature in April, if only because it would be nice to have something to look forwards to besides egg yolks for those of us who don't care about stats that much (me!). To keep with the springy theme of the event, perhaps something like 'powder blue guinea fowl?' Powder blue is a color mutation of the helmeted guinea fowl. Just like the GMO cow, it's a man-made mutant. 'Pastel chicken egg' could work too.

In summary, this item would have a 50% chance of ensuring the dam's base genetics pass to her cub. It would not allow her to pass anything she ordinarily could not and the pass rates of rare bases would remain rare relative to other bases within their genetic categories since the item essentially lets you skip the rolls for color, shade, gradient, and rarity, but does not boost the actual pass rate of any base itself. It would stack with the crunchy worm and it could possibly be a good fit for the April event. Unlike the crunchy worm, it could not ensure 100% passage of anything from mom, but it would allow mom's base to pass and still combine with dad's markings, mane, or eyes.

If anyone has any suggestions, disagreements, would like something clarified, could clarify relative pass rates of heritable bases for me, etc., please comment!