In both cases, more information is available, but Proc::ProcessTable does not know how to retrieve it.

You will need to find where ps(1) gets its information and then submit patches for Proc::ProcessTable. A good first place to look is /proc/<PID>. There may be additional files in there with useful information; this is certainly the case on Linux. Unfortunately, I don't have Solaris or HP-UX close at hand, so I can really only speculate on what you will need.