So here's what happened with the K120. I think I got 99%+ of the foam out of the voice coil vent. And the speaker sounds great. Hopefully no related problems will surface down the line. In case someone finds themselves in this situation, here's how I went about extracting the foam.
I set the cabinet on two chairs with speaker facing up so that there was about 2 feet of room underneath the voice coil vent and crawled underneath. With a flashlight I could through the back of the cabinet, to the vent, where 2 scraps of foam were sitting on the vent grill. These seemed to add up to the one third entire size of the vent- which is a circle about the size of a half dollar. I laid underneath with a flashlight, bent paperclip, and a pair very thin forceps and was able to pull the foam out in very small pieces. The foam crumbles very easily, so I used a vacuum cleaner every so often to suck out the dust & small pieces. (I was careful to not cover the entire vent opening with the vacuum- I thought too much sustained and complete suction might cause damage by moving the speaker/coil too abruptly.) Then I turned over the speaker a few times and checked again. 2 much larger pieces of foam were now on the grill-these added up to about three fourths of the size of the grill opening. I repeated the above, and was able to remove all traces of these pieces. I checked three more times (turning the speaker, getting underneath, etc), but no more foam pieces appeared.
All together, I the pieces I was able to remove added up in size to just a bit more than the size of the grill opening. So, I think I got it all, and before it had a chance to crumble and work it's way into the small spaces to cause coil rub. What may have helped here is that the speaker had been stored unused for a long time according to the owner. It was in a 10 watt harp amp, that he said was never used much. The owner lived in a very dry area, with low humidity (I know this because I live in the area), which may also have helped to prevent any existing foam dust from collecting into sticky clumps that could cause rubbing problems. Thinking back on the day I bought this, I may even have jarred the foam loose by doing the battery test on the speaker. That caused a big cone displacement when the leads were briefly connected.
For additional info on this and related problems, I'm including these RUKind subject headings for related JBL posts that I read through that might be helpful to others in this situation. The first one is already mentioned in this thread, and is also a topic at TGP, with a lot of good comments there as well.
Burned on K120 w/voice coil rub - what are my options?
Blown K120 blues
Dented dust cap?
Re: Whats In The Box ?
Q's on JBL D120Fs.