Agreed that there are so many different Geddy sounds.
His flamenco style wasn't always the case. Although I'm not positive when it first appeared, the song Animate on Counterparts is the first time it is actually required (IMO) to achieve a similar performance.
Also, while he started using Sansamp around the Vapor Trails (and My Favorite Headache) time period, he didn't use it previously as far as I can tell.
What he used live versus what he used in the studio back in the day takes a lot of digging and guesswork.
It seems the Moving Pictures and Permanent Waves (along with Signals IMO) is considered the pinnacle of Geddy's sound. For that sound to my ear it is almost better to go with something like a Sansamp VT Bass.
Right hand attack is very important as has been noted. He really thwacks the strings downward at times. Your action needs to be low, but not paper thin. Otherwise you'll choke out all possible sustain. He also moves his right hand around a lot searching for the sound he wants. It is usually over or in front of the neck pickup, but not always. Also, from what I recall, all of Permanent Waves and I suspect most of Signals was the Ric, whereas Moving Picture was about 50/50 Ric and Jazz.
Also, if you are playing Rush songs and learning from tabs, most of the time they get the majority of notes right but many of the positions wrong. If something can either be played on the G string second fret or D string 7th fret, it's a good bet that he actually plays it on the D string. Your starting position might be middle finger on low G, but his starting position is generally A string fifth fret.
This may not be accurate or useful, but it's all I got.