Hello again. It' be terribly irresponsible of me to not update this post. According to the Trojan-riding scribe at Boi From Troy, the Song girl with her backdoor blown open is... Song Girl Megan. His conclusion comes from a highly regarded "source".

From BFT:

It's Megan....here's how I know. Alison and Ali are first years so they wouldn't be second in the 'step-off' line that's reserved for older members...kim and natalie (the two song girls behind the twirlers are the captains. Also I know know them all, and from the pics I know who it is...

Congratulations to Awful Announcing and commenter Kenny Loggins for being the first to presumably come up with this answer. Your hiney-sleuthing skills are not to be underestimated.

As you were.

Song Girl Mystery Solved [BoiFromTroy]
Megan! [USC Song Girl Official Site]
Deadspin Special Report: Another Vixen Thrown Into the Mix [Deadspin]