Richard,
I am not sure I am following. MouseToCell isn't an event in TDrawGrid either, it is just a method that you can implement as above. You have OnMouseDown, OnClick, OnMouseMove, etc events, which happen when you click, move the mouse etc. But MouseToCell isn't an even't (when would it fire?). It is just a method that you can call to convert x,y coordinates to a cell.
What is what you want to do? To show a popup menu you would assign the property "PopupMenu" of the FlexCelGrid (or the drawGrid). If you want to do customizations, you have events like "OnPopup" in the TPopupMenu component that you assign to the grid. If you want even more flexibility, you can handle the event OnMouseUp and do everything manually.