That can be done however, you would need to draw the grid in owner draw and keep track of the merged cells.
when a ItemDraw event takes place, search the list and if the item is the first in its group then you need to expand the drawing rectangle to include the other merged cells to the right and down.
With now a new resized rectangle you can now draw the merged cells as if they were one cell.
when the other cells come in for a draw session you first determine if they belong to a ground, if not, then draw it else if they do then determine if the cell is the first upper left , if not, then ignore the drawing and don't allow the inherited code to draw it.