on a Linux box I'd definetly make use of the marvelous /proc virtual filesystem.
Unfortunately does HP-UX lack such a feature.

I think HP-UX offers a /dev/kmem for kernel space queries. But since I never accessed that special device deliberately (I'm also hesitant because I wouldn't want to tamper with the kernel of a production server while I don't know the implications of privileged access to the kernel data), I simply don't know how to retrieve data from there.

I think you are right that probably calling lsof and parsing its dump would be the easiest option.