Solution: Use Ethyl / Denatured Alcohol to dissolve it off. I've used this solution on two bags now and it had worked amazingly.
Bag 1 - Thermarest stuff sack: It was so sticky I had to peel it apart. I turned the bag inside out and put it in a jar with rubbing alcohol 91%, let it soak for about 24hrs then pulled it out of the jar & used a scrub brush on the sticky part. The old coating either went into solution or balled up and came off.
After a second soak/scrub, I threw it in the wash on hot/quick with laundry powder to help wash away the rest of the solution. After it air died, it was completely tack & odor-free.
Bag 2 - older Dana Design pack: A larger (60L) pack with many facets, with the same issue. First, I took off as many of the straps, foam & stiffeners as I could. I put it in a trash bag, then put that in a bin of water. This way, when I poured Denatured Alcohol into the trash bag with my pack inside, the pack would get soaked/saturated without needing 3 gallons of alcohol. The snap lid of the bin also helped hold the pack down, preventing it from floating on the surface of the water.
After it'd soaked for 24hrs, I put on rubber gloves & pulled the bag out for agitation. I used the lid of my bin as a tray outside, to capture as much of the alcohol as possible, so I could pour it back in the trash bag for round 2. I scrubbed the pieces of fabric against each other or used a stiff brush. When I was satisfied, I hosed it off, threw it in the wash & air died. It's completely tack free.
I opted for the denatured over rubbing alcohol because it's mostly ethanol (renewable). As a solvent they worked about the same. I didn't try diluting them. Hope this helps!