Almost guaranteed that the suction cup won't hold it. The only suction cup design I'd trust to hold in a harness is the Fun Factory one, as used on the Tiger.
I haven't used the Rascal the Master dildo, but I think you might have some problems with it staying in a harness reliably. The balls will hold, but the lack of a flange at the top means that it will probably ooze out of the harness during use. You've probably noticed the "O-ring compatible" selection under the "harness compatibility" category in dildo search, but you probably have also noticed that there isn't anything over 10" available.
If you can put up with readjusting every once and a while, and having a bit of difficulty mounting the dildo on the harness, the Rascal the Master would probably work OK, especially if this is an experiment or one-shot deal, but if you want a dildo that you can use repeatedly in your harness you'll probably be happier with something else with a harness flange. Unfortunately I'm not sure exactly where you can go to find it.