I'll offer an alternate opinion - I analysed not a song, but an album in my Extension One essay and got 25/25 in the HSC. Similarly, a friend of mine achieved 94 in Advanced English using 'Bittersweet Symphony' as a related text.
The difficulty of analysing a song is differentiating it from a poem - you need to discuss the lyrics alongside with elements of musciality and layering (I'm 8th grade in piano which kinda helped out a bit). Analysing a song as simply a poem is an incorrect approach in the HSC, and will be detrimental to your analysis. I believe that doing a song is great because it allows you to stand out - there are around 50,000 who do Standard and Advanced English. You want to pick a related text that isn't over done, something which makes you different to everyone else. Remember, examiners are people as well, and even though they have a set of criteria, there are definitely subconscious biases in play. Doing a song is quite rare, and will allow you to capture their attention.
To those people who say that a song isn't that substantial, if you have two related texts, each related text should only be around 200-300 words long. If you have one related text, then it's around 400. Whilst 400 words is a bit of a stretch, I would argue that you could analyse ANYTHING and make it 200-300 words long. Heck, you could probably do it with a single black pixel, given a slight amount of context. Anyway, you'll definitely be able to write out an analysis of that length.
If you'd like to look at an example of a song analysis, my sample for my Extension One essay (in the link in my signature) is the beginning of my paragraph.