The functionality is already there, but it is in the player view. If you open an activity, on the right side you'll see Details, Splits and Player tabs. Click the Player link. Once you're on the player page, you can select what data the graph displays (only two at a time). Select Heart Rate and Pace. Moving your mouse around on the graph will show Time, Distance, Heart Rate and Pace values as a tooltip.
If you use the play/rewind/fastfwd/pause buttons, you can look at the bars below to show all of the captured data for a given point in time.
