Smart parenting a figures BODY actor (only) has always worked for me (only tested in P4 & P5). Works just the same as in a prop, eg:

actor BODY:1
 {
 [etc etc]
 smartparent chest

The PROBLEM comes when I delete either the parent or child figure. Poser tends to hang or crash when either figure is deleted. because of this I sometimes use smartparent in my own figures, but strongly recomend you don't use it in any distributed files, no one will appreciate a file that can crash Poser.