Can TCAS be used to confirm "traffic in sight"?
No, "in sight" or "visual contact" means seeing the actual traffic visually, with your eyes. When you report "traffic in sight," then the controllers are allowed to issue a clearance such as "maintain visual separation" or "follow that traffic, cleared visual approach...". They can't issue those clearances when you only see the traffic on TCAS. In fact, having the traffic on your TCAS display doesn't really help them. 

The only time that I'll even mention that I have the called traffic on TCAS is in a situation where it may save the controller extra transmissions pointing out the traffic "now at 2 oclock moving to 1 oclock" etc. If he knows that I'm getting bearing from TCAS &amp; will call it when I see it, that may save some radio congestion, but until I see the traffic and report it in sight, he can't clear me for anything based on visual contact.

TCAS is a great tool for collision avoidance &amp; for awareness, but it isn't designed as a "follow that traffic" tool.