I have seen this done before on a Weber Kettle Rotisserie, and it's definitely doable with a suckling piglet. The advantages with the rotisserie is that you can utilize the full diameter of the grill, and the rotisserie attachment gets the piggy down low enough that you don't have to worry about the lid.
The vast majority of cooks on our Weber Kettle have been indirect. I like to bank coals just on one side (like you plan to do) with a foil pan under the food. Make sure you place the vent on the side opposite the hot coals so heat is drawn across the grill. Using this method it's amazing how even the heat is.
Dave's mention of trussing and balancing the piggy is important! Here's a video that shows how to truss & balance it for a Kettle Rotisserie: