Your right it is dangerous, oxygen cylinders can explode, very nasty.
If its a disposable then you'll have problems getting it filled, esp with another non standard hole bashed in the side, cylinder fillers have to check the bottle has been pressure tested before they fill it, and they certainly will not refill a disposable.
That said, if your gonna fill it yourself or something you could fill the bottle with water, and then drill the hole while its still full.
A t piece and another pipe off the top is still a much better and safer idea.
Make it rather than buy it.