I use a powerhead to diffuse my co2. It is a pretty effective method I think. I too would be worried about injecting the co2 into the canister filter.
I don't have experience with subways, but I think Smitty's idea of a cab for those trips is good. It would be worth the money IMO. You shouldn't have to refill that often...unless you develop a leak, so it would be maybe an every 6 months deal, or longer.
I have a drop checker and I think that is a great investment. Definitely something I recommend.
