I cant see why it would be worthwhile. Firstly Im doubtful its legal to use a scuba tank for a non-breathable gas unless correctly marked (then again Heliox and trimix are used...). Secondly, the din or yoke valve is a different thread hence making fitting a CO2 valve near to impossible and thirdly, if you were to use a normal scuba valve, a scuba first stage regulator is not easily adjustable, even on the LP side, to get the pressures that are required for dispensing and/or carbonating beer. Remember scuba tanks are at working pressures of 230bar (3300PSI) which is at least 10x the normal CO2 cylinders.
Fil