Once again thanks BogdanH. I rummaged around in my collection of oils and greases, and found a cartridge of food safe silicon grease, NLGI grade-2. So, now armed with a sense of "I can do this", I dove into it today. I had opened the keyboard before about 1.5 years ago, but reviewed that video again (I discovered that not as many screws need to be removed to separate the keypad assembly from the chassis as was indicated in the video).
Removed a section (octave) of keys (which had the most rattle), then tested the existing grease (just with my fingers) against the grease that I had, mine was just slightly "thicker". Decided to use it. I removed the old grease (which had started to liquify is some areas) from each key guide slot with toothpicks and cotton swabs. I also noticed that the factory application of the grease was quite inconsistent, some key slots were covered and some had barely any. I re-greased each slot carefully, then moved on to other octaves. When done, reassembled the entire keyboard and tested it. Keys felt like they were when new, no more rattling, nice slightly dampened feel. I was happy, then.....
I found one key that had lost its "touch sensitivity" (the lowest G key), even the slightest touch sounded like I had hit the key hard. Oh NO!! Opened it all up again, removed the affected key octave, AND the silicon rubber membrane (this has contacts that touch the circuit board, one for each key, similar to some calculators). Cleaned the circuit board and underside of the membrane with Isopropyl alcohol (IPA) and cotton swab, let it thoroughly dry. Placing the membrane back into the circuit board is not easy, I had to fashion a metal probe of the right diameter to press the membrane "nubs" back into the circuit board (glad I have a shop full of tools to do this). I did some minimal reassembly to test it. The G key worked perfectly again, and all other keys were still good. Fully reassemble and test. Seems good.
One word of caution about placing screws back into plastic holes. Don't use a power screw driver, use a hand screwdriver. First "set" the screw into the plastic threads by turning the screw backwards (CCW) until you feel (and/or hear) the screw drop into the threads, then tighten slowly. If there is resistance, then back it out again and retry the same procedure. Finish tightening until just snug and no more, because it easy to strip the threads in the plastic hole.