of course it depends on the situation, but another thing that, at least in my past, used to work (i never went by myself to a club, but many times i split from the crowd and started some alone-sessions) is talking to the bartender, if you don't feel like wandering around.
In your case, if a bartender is available, you may wanna tell him/her something, some small talk; what happens is that sooner than you think you'll be interrupted by someone ordering a drink, and while he/she/they order, you can definitely approach them: it's easy to just even say "that sounds definitely better than what I'm having, I should try it!" or "nice ring, looks like mine..." bla bla... you won't need any social proof, since you were already speaking to the bartender (which may even play your game as a social proof, like "he's cool, he may know the bartender..." kind of stuff).
That way, when they get drinks it's soon done: if he/she/they don't invite you over, or even better don't stay there to chat (thing that usually happens), u can say "what about i finish my chat with him (nod to the bartender), get the *** (above mentioned drink) and reach your table to have a toast?".
that is just an example of how it may work. and if you have a.a., talkign to the bartender should take it away since YOU KNOW HE/SHE'S GONNA TALK TO YOU FRIENDLY, ANYWAYS, so there's no a.a. breaking need!
other way (totally cooler)?
pick a n-set (let's say 3) and open up like "hi, I'm just new in town and I couldn't help noticing you're 3; hell did you know that statistically the best and most fun nights happen when a group is made of 4 ppl? Let's join forces here (and propose a toast)"...
Just to say, if you have "balls" to go to a bar alone and try to open to a girl sitting there, you should have balls to use as much immagination as possible, it can be totally fun.
PS: I am new to Jhb, if u wanna go out sometimes and sarge/party/have fun, I'm up to it!