I'm currently reading through the new Perl & XML book, and I must say that XML is a lot more flexible and useful than I'd have imagined, and it seems like it'd be suited for this task.

You could even design a simple DTD for it, assigning things such as a <dialogue>tag that needs attributes of character - you could even split up the acts into their own parts of the tree. Here's a quick ad-hoc sample that comes to mind (just from somewhere in the middle, not including a DTD or the root element)

<act number="2">
  <stage-direction character="Fred">
    Enter, stage right
  </stage-direction>
  <dialogue character="Fred">
    Man, it's hot out there!
  </dialogue>
  <stage-direction character="Fred">
    Wipe forehead
  </stage-direction>
</act>
[download]

It could just be that I'm loving this new book, but it seems like XML would be the markup of choice here, and you learn a lot in the process.

~Brian