I suspect they can be filled (someone did it the first time) some way, but also think that if I had to ask how, then I shouldn't be doing it. Understanding what the limits are in the container to be filled would be very important, and understanding what the potential is for over-filling and over-pressurizing would have an impact on your safety.
The apparent reason for the OPD valves on the 20# tanks was because people were over-filling them, making them potentially dangerous. I'm not 100% buying this reason, and think it was a move to replace all the old tanks that may have deteriorated due to age and abuse. But it gives food for thought.
At under $2 per canister (thought that was what I have been seeing), seems to me that filling them with any degree of hazard might not be worth the trouble. But I'll try not to speak for you, and wish you luck finding out how safe it might be to re-fill them.