The trouble is with places charging a flat rate of say £50/fill regardless of car, is that most cars take 6-700g... then we rock up with a P38 needing 1200-1380g. Suddenly they're barely making any money taking cost of gas and their time into account. At the same time... that's not my problem.
The DIY refill things I'm not a massive fan of because in the UK, they can't legally contain actual refrigerant anymore. So long as the pressure on the low side doesn't end up massively high, it should be okay. But it can't beat weighing it in. Equally.. if the system is empty and has air in it, you're filling a system with an unknown 'refrigerant' as well as the air, which will have moisture in it and emulsify the oil... all ending in a mess. Pressures probably all over the place too. Then you have those with 'stop leak' additives in, which I've seen quite a lot of stories of TXVs being blocked up as a result, needing evaporators pulling out to replace etc.
I'm not sure on the R12 idea - I believe propane is mostly compatible in properties with R22, and R290 is basically just propane with a very low moisture content. Just has the slight downside of the old flammability issue... although the new R1234YF is too, so I guess car manufacturers aren't that bothered. I know a lot of us drive around with LPG plumbed up all over the place, but to me you're much more likely to end up with a big leak on a condenser when a nice pointy stone hits it or you have a collision with something in front....
I think the other potential concern is if you ended up with a leak where it leaked into the cabin area - possible given the evaporator is linked to that area by its job. Also I've seen the results of a compressor left open to the air (granted it was for an extended period due to a hole in the pipework) and can confirm it will knacker them completely. If the photos below work you will see what I mean.