Interesting thread - and timely, I have just built an O2 kit for diving use here in the UK. I am DAN trained and have both O2, CPR, O2 enhanced CPR and AED certification but had a real problem getting 100% O2 at a reasonable price (most place wanted $200-$300 for a cylinder).
My solution was to buy an variable rate (medical) oxygen regulator that will supply a constant flow between 5 and 25 litres per minute to a mask, or supply a demand valve, (this regulator has a pin index fitting) then I bought a pair of adaptors - pin index to bull nose and bull nose to DIN adaptor. This gives me the best of all worlds. I can connect it to any cylinder, dedicated medical (pin index), industry bull nose cylinder or diving tank with DIN, or 'A' clamp with a standard yoke adaptor.
I have a 5 litre O2 clean diving cylinder (DIN fitting) with 100% O2 filled to 200 bar, but I can also connect to any diving cylinder with Nitrox (lots of people round here have high O2 % deco gases with them) if I need to if mine were to run out. The dives shop that filled the cylinder knows me, gives me O2 fills against my O2 provider card, and trusts me that I'm not going to put a diving regulator on it and use it for deco, which of course I could easily do configured this way.
To finish the kit off I am just looking to source a suitable size peli case at a reasonable price.
Regarding liability the only thing I was told quite forcefully was that I should only ever use O2 in a diving emergency, not for any other collapse etc. because there are some medical conditions (mainly lung diseases) which administering 100% O2 can make worse, but these conditions are not likely to be suffered by divers (or they have lied or their medical certification). The kit will be available in the back of my landrover 24/7 so whenever I am diving it is there, and it will be easy enough to lift it out and onto our club boats when we take the rhibs out. - P