All I can say is, try it and see. Non- castille soaps (ivory is a castille soap) have additives which can cause them to get 'gloppy' when you melt them. I would grate it very finely, and melt it over VERY low heat.

If Dove doesn't work, try a milk-based soap.

Your library may have a book or two on making "Hand-Milled" soaps.

All you can do is try - if it melts smoothly, it will work in the loofa.