There is an idea that I think could be made to work. I don't have time to play around and write it at the moment, and what follows is probably a bit advanced for you, but someone else may grab the idea and give it a go:
You can lay a Forms Image over the cells you want to potentially have the tool-tip for. You then set the Backstyle of this to fmBackStyleTransparent. A forms object has the MouseMove event. With some care and a fair amount of coding you can work out when the mouse is over a particular cell and then bring up a shape containing your text that tracks the pointer.
Take a look at this but don't be mistaken, this is not 'nearly there' to turn it into what you want requires quite a bit of work, but in theory it could be done.
Private Sub Image1_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
.Left = X
.Top = Y