I'd be likely help you out, but I'm kind of an uber trail angel. Just last month, I housed and fed a BPL moderator and her family for two nights and then arranged to have two 30-pound food drops done, by air, for free, 200 miles past the last road-accessible point on their journey this summer. Which included some 99% IPA transferred to a recycled, 8-ounce HDPE container.
I'd be most likely to help you out if I was packing out that day and still had some butane/propane mix in a canister. Then, if you had an canister-to-canister adaptor (which I haven't seen mentioned on Jim's website and I haven't found, so I'd have to make one), and some chemistry / chemical engineering knowledge about vapor pressures, etc, you'd be all set.
Without spelling out all the gory details, the rub is that the donor canister has to be warmer than the receiving canister. You can make the receiving canister colder by burning off some fuel or leaving it out overnight or getting it into cold water. It is easier to make the donor canister warm - stick it in your underwear, leave it in the sun or (quickest) use your Bic lighter, although many people shy away from the flame-around-fuel-tanks thing cause of all those scary signs the fire marshall puts up. Weenies!
But if you had that procedure down, then you could benefit from someone who doesn't care about the last 1/4 to 1/3 of a canister. And you could combine two partial canisters into one fuller one.
Here's an easier strategy that would be more successful: Carry some $1 bills (1 gram each). When someone else is outbound, offer to buy their partial canister(s) on a pro-rated basis. Don't we all have a ton of partial canisters at home? They could avoid having one more. You saved a trip to town. They saved a bit of weight, bulk, and recouped some of their fuel cost. If you worry about the bulk of the accrued empties, clobber it with a boulder. If you are bothered about the weight, make part of the $3 deal that you buy the partial and they pack out your previously emptied (and flattened?) canister.
Often when you expect the world to support you, the world shakes its head at you. Often, when you offer to pay for small things, they are given freely.
Why/How do Jenn and Hig score a lot of freebies? Because they are traveling 800 miles totally human-powered, half of it trail-less, bear-infested wilderness with a 4-year-old and a 2-year-old and blogging about it as they go. You and I will never do anything near so cool or challenging, so we shouldn't expect as much assistance.