Lee the answer is YES!

Graham is giving you good advice, although depending on the type of ballad it could still have a rhythmic bass line. You would have to edit the bass track and replace any chromatic sequence with a sustained note that retriggers on demand. A voice like Contrabass or Strings would be appropriate, depending on what you're going to use it for.

There's no "magic" to a free play style. The very first one on the Tyros (3?) was Ethereal Movie, and it played just fine on my PSR-3000. As you and others have surmised, they are characterized by the absence of drum and percussion sounds and lack of a rhythmic beat.

I've made very simple "free play" (rubato) styles for Korg and Roland as well. It gets tricky on a Roland because not all sound samples retrigger reliably if the keys are released and then pressed again in the middle of the style pattern. Unlike almost every other brand, Roland doesn't have a user-accessible retrigger parameter. You can force it to retrigger at measure 1 beat 1 by activating Sync Stop, or you can choose another internal instrument voice that implements the attack and note on messages differently, so the sounds are retriggered by the style engine. I've found Jazz Organ type voices to be excellent in this regard.

I recently experimented with a Casio. I was surprised to find an on-board style called String Quartet that for all intents and purposes is a free play style! They are very useful for pieces that have a time signature change, and also for practicing!