XML is a good candidate, but actually overkill for the purposes I was looking to achieve. Basically, all I needed was character names are all caps, scene headings have hashes, and stage directions are free text. Put a handful of other niceties like title pages, casts, and page breaks, but never anything so complex it needed additional markup.
# INT. JOE'S BEDROOM, DAY.
JOE
This is my dialogue, yeehaw.
JOE exits, and we know JOE is exiting because this is a stage direction, and we know this is a stage direction because it is free text and doesn't satisfy any other formatting requirement.
# INT. JOE'S BEDROOM, DAY.
JOE
This is my dialogue, yeehaw.
JOE exits, and we know JOE is exiting because this is a stage direction, and we know this is a stage direction because it is free text and doesn't satisfy any other formatting requirement.