So, did some more research. Disappointingly, I don't think Max Dog Brewing's understanding is that good. Medical gas cylinders do seem to be fitted for CGA-540, but if you're going to an industrial gas supplier, here in Canada it seems like you're getting a CGA-580. Luckily, that seems easy to get. Here's a sample:
BTSHUB Argon CO2 Flowmeter and Regulator Gauge Mig Tig CGA580 0 to 60 CFH Welding Gas Flow Meter, Full Copper
Annoyingly, output on that is cubic feet per hour, rather than liters per minute. It maxes out at 60 CFH, which is about 28 L/min, easily managing the recommended output of 20 L/min.
Sorry this is turning into a bit of a journal, but perhaps seeing this part of the process will be helpful to some. Exit bag is definitely a bit of a nerdy way to CTB.
More math: the typical cylinder I hear recommended is 22 cu. ft. Converting back from L/min to CFH, it seems that the recommended output will empty the cylinder in about a half an hour. The bus should have pulled well out of the station by then, so it seems like that's plenty. You could get away with less, but for peace of mind, it's a sensible amount without getting ridiculous.