I've definitely seen the benefit of saving certain ingredients to add in after saponification. During the winter I was using a soap that was the same recipe that I use now, except I dumped everything in at one time. The way I prepare it now is to save a portion of cocoa butter to add in right before plooping into the mold, and all the dry cracking dehydrated skin I had from my soap and my husband had from the store-bought liquid shea butter soap he was using healed up and felt really soft by the end of the week. It stands to reason that other ingredients can have similar effects. (And as Maria said, an hour later and my skin still smells delicious.)
The effect I would hope for it to have would be for some amount to stay behind on the user's skin. Hyaluronic acid can hold 1000 times it's weight in water so it's fantastic at preventing dehydration of the skin and keeping it soft and supple. It's present in everyone's body and used by doctors to relieve osteoarthritis pain by injecting it right into the joint. (It acts similar to the missing synovial fluid.) It's also used by plastic surgeons as an alternative medium for lip injections. As more is learned about it it's becoming more popular for beauty serums and lip products for it's ability to keep skin moist and healthy.
Okie epidermatology lesson over
@Maria: Yeah it is pretty expensive :/ I was thinking of it for a more designer bar though. I've kind of been feeling like leaning away from making soap that's low priced and readily available to everyone... rather in a few years maybe shifting focus to rare expensive ingredients from around the world and aiming towards the snobbier parts. Of course not for a few years though
ETA: Also it would be better marketed as part of a kit, or set, with other items like lotions etc. that contain hyaluronic acid. People tend to think that one ingredient a lot is better than several unmatching ingredients
Ah, and before anyone jumps on me I will take into account if someone uses every product at once what concentration it will end up leaving on the skin
Also, hyaluronic acid will not be the only beneficial ingredient in there
Sorry if this seems overly defensive but this forum tends to like finding holes and ripping them open...