Please build the current 3.6.2 for EL-5. The current packaging repo does not build as-is, but either the build rules could be modified to compile a reduced feature build of Octave for EL-5 or we could patch the source to compile fully. The two parts of the build that do not work on EL5 are the curl library interface and the makeinfo command generating the doc-cache file for the help. I am working on a set of patches to ensure Octave compiles and works correctly with the curl library found in EL-5. I think the doc-cache file is only used by one command in Octave, so if we have to bypass that minor step for EL-5 we still have a 99% working Octave build.
This would go against the EPEL and updates guidelines. Besides, I looked at updating the EL5 branch to the 3.4 series, but soon found out that a lot of the requirements such as gnuplot would also have to be updated.