I do know meadowfoam or jojoba increases the shelf life of oils such as grapeseed and rosehip oil in my lotions and salves. I use meadowfoam or jojoba as a high % of the total fat in the lotion, however -- maybe 30% to 80% of the fats in the recipe.
You don't mention what % of meadowfoam you'd add to the grapeseed. I'd think if it was a fairly low %, it might not do too much. But I haven't tried it to know.
Honestly, if you want a really effective additive to protect your fats during storage, you're better off using the antioxidant rosemary oleoresin (ROE). You use it in tiny amounts, like 0.05%, and it does a very good job of protecting fats from rancidity. I would also store grapeseed in the fridge as well, just to be on the safe side, as you already are doing.
As far as extending the shelf life of soap, some of the research I've read says you're better off to use a chelator such as sodium citrate, ETDA, etc. if you are going to use just one additive. Rancidity in soap is often triggered by metal contamination, and that's what a chelator controls. An antioxidant would be a close second on the list, and in that case ROE is a good choice.
