That would suck if it started leaking!!! Especially because as it heats up being on your back, the gas will expand. The aluminum or SS fuel bottles for camp fuel would be much better for throwing in a back pack.
I used to re-use those little ~4oz bottles of chainsaw pre-mix and carry those around on my KDX. I'd pull right up to the pump, dump in my oil, pump the exactly one (or two) gallons, then shake the bike around to get it to mix. Actually, I'd try to get a little gas into the oil bottle to get all of the oil out. Never blew up the KDX!!