The Locomotor spell is used whenever a wizard needs to carry a large load at about a walking pace. But what would happen if the wizard were to cast "Locomotor me" or "Locomotor [casters name]"?
We don't know, but if anything, something might happen to their legs...
The Locomotor spell is only used a few times and always to move something large (as you mention);
- Tonk's uses the spell to move Harry's trunk. (Order of the Phoenix, Chapter 3)
- Flitwick used the spell to move Trelawney's things back to her room. (Order of the Phoenix, Chapter 26)
However, "Locomotor" is also used in two other spells, both which effect the legs of a person;
- Locomotor Mortis, the Leg-Locker Curse (Philosopher's Stone, Chapter 13)
- Locomotor Wibbly, the Jelly-Legs Curse (Philospher's Stone, Chapter 13)
- The final time we hear "Locomotor" is when McGonagall uses Piertotum Locomotor to animate the suits of armor (Deathly Hallows, Chapter 30)
So, at least half the time the "Locomotor" spell is used it is against another wizard, it uses a second spell word in conjunction, and affects the legs.