What I was planning to do was display a JS link which says "View phone number", which when clicked will reveal the phone number, while in the background a jquery ajax call will record the request.

This would only affect a small percentage of the content of each page, but if I use it everywhere (which I need to for it to be much use) it would affect several thousand pages.

Hidden text has a bit of a bad name. I've never used it, or wanted to, as an aid to 'optimisation' and can see why it could be looked upon with disfavour by G and other SEs.

Is a small amount like this OK? The other possibility I'm considering is an iframe, but it's much less elegant technically.