DocBook is not what I'm looking for. It uses SGML markup... .However, I'm wondering if a two-stage approach might work. I can write a simple Perl tool to take an easily-typed markup and convert to formal XML, and that's all it does.

Yes, basically DocBook was sgml stuff. Fortunately since Docbook Version 4 there is also a XML-DTD and a XML subset of Docbook called Simplified DocBook.
You could even convert your easily-typed markup to Simple DocBook. I don't want to talk you into some format like DocBook with a rather steep learning curve, but just want to show you an alternative.

Hanamaki