If I recall correctly Leslie was there in a "guest star" capacity and just played on a few tunes. I would suspect that the number of dates he played with them was limited.
I remember not being all that thrilled to see BTO as I wasn't really into them and I was TOTALLY into Van Halen that year. However, BTO were featured on KSHE's lunch program the day of the show, and the interview with the guys had me at least interested. Also, though my dad was into Mountain, I knew very little about Leslie West.
The atmosphere was thick with anticipation as show time neared. This was the first time Van Halen had come around with Sammy Hagar and there were banners everywhere that read, "David Lee who?" and alike. I don't know how the rest of the country felt about the change but St. Louis has always been Sammy's town!
As soon as BTO came on stage, I knew this was going to be much better than I thought. As it turned out, I knew just about every song they played; including "Mississippi Queen" by Mountain. And of course, Van Hagar totally rocked the Arena that night!
I found My Concert Archive while trying to find proof that Leslie West was indeed touring with BTO at that time. I recently bought a copy of Live! Live! Live! by BTO and it has no mention of Leslie West on it anywhere. I know this came out not long after the Van Halen concert and it contains, "Mississippi Queen". Had West left the group by the time the album was recorded? If so, were BTO still performing the song without him? It just doesn't make sense to me that "Mississippi Queen" would be on the live album documenting the tour he played on, and there be no mention of him anywhere on the packaging.
Does anyone know any more about this? If so, please let me know.