> Recently, my two-year-old son has been taking off his diaper at nap time. ***It's usually full and dirty and makes a big mess.*** 

If the diaper is terribly full and dirty why exactly do you want him to keep on wearing it? It might be utterly uncomfortable/wet, that might be the reason that he is trying to take it off!

My advice is to not let him continue wearing a diaper which is so full and dirty. Before he starts becoming uncomfortable, simply change the diaper.

Observe what happens when he wears a clean and fresh diaper. Does he still try to pull it off? If yes, then duct-tape may be the solution. If not, then have mercy on him and change the diaper.

Making him unable to take his dirty diaper off by tape is cruelty IMO.