Our policy is to keep the size of the control as small as possible and not to bloat it with features which are used enough rarely. The existing rectangular built-in cell tooltips work ok in 99.99% of cases as built-in grid tooltips which do not require any coding, so we do not plan to extend this functionality.
If you already use hTooltip as the main tooltip provider everywhere in your app, then it's a wise solution to use it as iGrid cell tooltips. The RequestCellToolTip event is the right place to suppress the default cell tooltip and display your own one. If you need any help with implementing this solution, feel free to contact us.