Wow, it's been four years since I started this thread and I'm finally shipping the chassis off to the body shop this week. One of the last items I had to do before sending it off was to remove these cable ties in the engine compartment. I found one that was a little loose and did as John Walker suggested to remove it.
For those interested, once I had it out I could see that it is uses a plunger style mechanism to expand the tabs holding it in place (see photo). Unfortunately there is no way to get under the clamp to push the pin back out. Of course you could simply push the pin all the way through to remove the clamp easily, but then you wouldn't be able to retrieve the pin so it would not be re-usable (unless you happen to have some spare pins laying around).
I still would like to find the part number for these as removing them by using JW's method does damage them slightly. And I've only removed one so far that was a little loose to begin with, so I suspect more damage on the remaining four.
Cable tie with pin retracted:
