Cecotropes are the primary protein source and a critical part of the rabbits' diet. Baby bunnies eat their mother's cecotropes both for nutrition and to enable helpful bacteria to colonize their gut. Generally the babies will eat cecotropes directly from the mother; in some cases harvesting cecotropes from a healthy bunny and including them in feedings is the only route (or syringe/force feeding). Some sources indicate that a rabbit will not eat a dropping that has fallen on the floor; while that may be true in some cases, I have found that is not true in all cases.
Harvesting cecotropes is easier said then done.
The recommended method is to put a cone collar on the donor/healthy rabbit, so they are unable to eat them directly, then harvest them from the floor. This may be where the myth that bunnies won't eat them from the floor comes from, but I would surmise that the whole cone collar, can't even eat fresh cecotropes frustration is probably stressful enough that the donor rabbit is not interested in eating them.
The second method is harvesting as found. If you notice a bunny with its nose to its tail, you can sometimes distract them and harvest them fresh. Occasionally, you will see them laying around (small, moist, in grape-like clumps).
This answer is primarily to address the second. Can I harvest and conveniently and store for a future need. It primarily focused on the survival and introduction of "helpful" bacteria.
Some sources recommend yogurt or freeze-dried, cultures as a source of L. acidophilus as short term solution to bacterial imbalance. This is not a constituent of normal flora in the rabbit and the pH balance of the stomach can kill it before it gets to the cecum, there is some controversy as to its effectiveness. There are similar conciderations with commercially available probiotics.
Rabbits have a significantly different digestive flora (cecum bacteria) than most other mammals. The stomach and small intestine have no native bacteria, they all reside in the large intestine and cecum. The majority being Bacteroides. E. coli & Lactobacillii which are common in most other mammuls are generally absent in healthy rabbits. Source Rabbit Feeding and Nutrition, by Tony J. Cunha & Peter R. Cheeke, Elsevier, 1987, ISBN 008057078X, 9780080570785
We want to get Bacteroides through the digestive system to repopulate the cecum. They are an aerotolerant, anaerobic organism (don't need oxygen, and exposure to air will not kill them "quickly"). Some sources suggest that the mucus covering of the cecotropes help protect the bacteria to get through the stomach alive in order to recolonize the cecum.
We have identified the mechanism of transfer from mother to kit, the challenges of the bacteria in surviving the trip from the mouth to the cecum. We know that type of bacteria we want to colonize with is Bacteroides. Now, unfortunately, we discover that they are relatively fragile outside of the body. The article Bacteroides Species: Maintenance of Laboratory Strains does a good job of describing the process to maintain a culture for laboratory study. It appears that due to this commercial products containing Bacteroides are not available. One site was found giving directions for creating an on going culture in the home for self inoculation.
In summary it seems that "fresh is best" is the solution. It seems probable that cecotropes that no longer have a moist mucous covering, have passed their use by date for potential Bacteroides recolonization.
Image of cecotropes (warning - potentially NSFW):