What does '麦上' mean in 小蛮腰 song?
A couther interpretation of 站在麦上顶你 is 站在麦词的立场上顶持你.

麦 is a short form of 麦词.

麦词 are the words of the rap music.

The MC stands the microphone controller.

喊麦 is the corresponding Chinese term in China.
The words are then called 麦词.

顶 means to support, to agree, to say yes, to give a plus (Google+), to give the thumbs up (Facebook) or to give an upvote (here, Stackexchange).
The similar terms are 赞 (Weibo), 推 (Twitter), and so on.

Therefore, 站在麦上顶你 may mean 用麦词支持你.
I support you from the standpoint of the rap words.
I support you with the rap words (so that you can continue ...).