Hear Homer and Jeremy,
Just in case you have not seen these, the CLAS12 software group and CODA group have done some of the work already to read EVIO files. I think these can be useful in further understanding the data format.
I create tar archives of the relevant directories of the current svn repository at Jlab and put these at:
You probably want to start with cedExport (does not have the sources), which contains some example EVIO data (in the data directory), and a program to browse an EVIO file. To run it, cd into the cedExport directory and run the appropriate script ( "sh evioview.sh"). The other program "ced" is the full event display program.
The source codes are in the other tar files, Jevio is the Java EVIO interface, bCNU is the code for the clas event display base libraries, ced12GeV is the code for the actual implementation of the ced (CLAS Event Display) program. This is the program that Dave Heddle talked about at the last collaboration meeting.
Hope this helps. When looking at the data format you will probably realize how primitive EVIO really is.