Here are a few more pages that may be helpful for you:
Patrick and Matt:
Overlapping Hierarchies / Concurrent Markup
http://www.sbl-site2.org/Extreme2002/
Just-In-Time-Trees (JITTs)
http://www.sbl-site2.org/Extreme2002/
Jeni Tennison:
The Layered Markup and Annotation Language (LMNL)
http://xml.coverpages.org/LMNL-Abstract.html
Michael Sperberg-McQueen and Claus Huitfeldt:
GODDAG: A Data Structure for Overlapping Hierarchies
http://helmer.aksis.uib.no/claus/goddag.html
These are probably good starting points for concurrent markup.
Jonathan
