Used to work in the aerosol industry. There are DOT specs to the canisters that are used, and can be transported. Any can with the rolled lip and standard aerosol valve, which is what you find on the gas blends canisters, is NOT rated for the pressures propane exerts. That is why the 100% propane canisters are thicker stronger steel, with more robust valves and safety release valves. It is all about the vapor pressure. And to the poster that said that propane and butane are not that different in size, you are right, there is only one -CH2- unit difference, but in a molecule that small, that is a huge percent increase in moelcular weight (propane MW= 44, butane = 58, 31% increase in MW), thus the large drop in vapor pressure from propane to butane.
Another hazard of this is over-filling. All liquified gas canisters should have "head space" to allow for expansion of the gas if it is heated. The hazard being, if you overfill the canister, there is no room for the gas to form, you get hydrostatic pressurization and the canister fails. It is industry standard to leave 15-20% volume of a canister empty for headspace.
I would also say that this device looks incredibly dangerous from the perspecive that the tubing used looks like simple PVC or tygon tubing, which IS NOT RATED for the pressures you are going to be transferring that gas at. You should only be using tubing rated for pressure, which will have a braided stainless sheath around the tubing core.
Another issue to consider as well, the DOT specs for canisters that are considered refillable vs non-refillable. Non-refillabel canisters do not have to pass results for long term use and are not spec'd to be corrosion resistant. When refilling a canister you are potentially introducing water vapor which can potentially corrode the can causing catastrophic failure. Refillable DOT canisters are made with this corrosion resistance in mind, and therefore are "over-built" to make sure that they can withstand some corrosion without failure, at least until the next testing period. ALL refillable DOT canisters have notices on them regarding occasional hydrostatic testing.