I was just using white lithium grease that you can pick up for about $5 at Home Depot for an 8oz tube. Currently, I'm experimenting with some higher grade grease thickened with calcium sulfonate that is supposed to better longevity than lithium thickened grease. It also has teflon in it, so it's also better for coating plastic parts. I am also contemplating putting this stuff in a small syringe (like thermal pastes for CPUs) and offering that as well. But realistically, I'd have to charge about $3-$4 a pop for a very small amount (good for 2-3 keyboards) to cover the costs of the syringe, time to fill it, and profit to make it worthwhile doing.
I think overall it'd be a good convenience, but very pricey as you are paying much more by volume, but you get enough for its intended purpose and still costs less than buying a giant tube that will sit in your garage for the next 20 years.