I don't know if it would work in a Mario RPG, but in other RPGs, I tend to name the party after Mario characters.
In Secret of Mana, the hero was Mario, the girl was.. usually an acronym for Princess (since this was before I knew she was Peach) and the sprite was Toad.
In Chrono Trigger..
Crono- Mario
Lucca- Daisy
Marle- Peach
Frog- Yoshi
Robo- Toad
Magus- Wario (or Bowser, depending on if I wanted to kill him in the story or not)
The Epoch (the time machine)- *I don't recall.. though in a future playthrough, I could call it Shroob or something..)
Those are the only games that I can recall playing fairly recently where you can change party member's names.