During a gap in the action at work this AM I found a youtube audio clip of a 1990 soundcheck jam featuring Neil Peart, Alex Lifeson, and Billy Sheehan.
At first I wondered if it was legit, as the drumming is very basic, the kind of thing that Neil would do in his sleep. But the guitar did sound like Alex's work, and the bass sounds like the faint memories I have of Billy Sheehan. The story was that Geddy was running late for soundcheck, and Billy sat in. This seemed plausible, as Mr. Big was opening for Rush that tour. This was the "Presto" album, and I saw them at the Palace of Auburn Hills, Michigan. Road tripped up from Notre Dame in the spring of my senior year. But I digress.
The youtube clip is broken into two parts due to length, with an abrupt ending at ten minutes. After the first part ended, I checked the almighty Google and, lo and behold, the story is true. From Billy Sheehan's official web page, here's a *.ram link
to the whole 14+ minute jam.
Not many bassists could sit in for Geddy, but Billy is a star and pulls it off well. Good stuff.