Basic mouse support has been in lnav for a little while now, but OS X's Terminal.app never had support for it, so I never used it much. Fortunately, with the release of El Capitan, mouse support has been added and it seems to work well enough for lnav.
To toggle mouse support on and off, press F2 in lnav. You can then click and drag to mark lines in the log or drag the scroll bar on the right side. The gif below shows the functionality in action.