The HP-UX sigar_proc_args_get implementation uses the pst_status.pst_cmd field which is limited to 64 characters.
There is a pstat_getcommandline function that has a larger 1k limit, but according to the this thread:
"A patched ps on 11i v1 uses pstat_getcommandline(). That is not available as a documented and supported interface until 11i v1.6 or 11i v2."
Patch posted to the hyperic bugs forum from Tony Ashburner: