Like most of Jaco's signature tunes, Portrait for Tracy is fascinating for many reasons.
I'm still trying to work out if it's a coherent single tune, or a technical 'etude', like JS
Bach used to write.
(Jaco took a lot of inspiration from Bach. Have a listen to Wanda Landowska's http://www.youtube.com/watch?v=cxPDiInNseY
, and then hunt out Jaco's version. And I'm surprised he never had a crack at slowing down and savouring the cello suites ( e.g. http://www.youtube.com/watch?v=1LcNA31fFSk
). Someone should do this in harmonics some time!
So, while there's a lot of harmonic freedom in it which allows the 'ringing chords' to hang in the air, and, as you say, give the tune it's emotional depth, there's probably a classical structure to it too.
In terms of interpretation, can I suggest that next time you sit down to play it, you try just holding your bass, and singing/humming (out loud) it instead of playing it with your fingers. Let the tune and the rhythms get inside your arms and elbows before you play it. See if you notice a difference in how you them play it.