I'd say that with Alex Lifeson there isn't much improvising. I think he's one of those guitarists who actually works out his soloing, so maybe 'improvising' is the wrong approach. I remember reading an interview with him in the mid 80's when Rush were at their peak and he more or less said that he literally works at a solo phrase by phrase, quite often at the time of making the recording. Great guitarist to have as a mentor though.

You should also check out Kim Mitchell from a band called Max Webster. They were very good friends of Rush and made an album with them called Universal Juveniles.