I'm thinking about adding basic HDR support into Octave image package. MATLAB references Larson, Greg W. "Radiance File Formats". http://radsite.lbl.gov/radiance/refer/filefmts.pdf with details on various HDR file formats and related source code. The source code from www.radiance-online.org has a permissive license which requires redistribution of the license acknowledments in user documentation (I'm no lawyer though).
Do you know if using such a library is possible in Octave and therefore worth a try?