I don't have experience or access to a DGX-660, so I can make a suggestion or two but have no idea if they'll work.
Idea 1: Set the Split Point to 021 (A-1, the leftmost key on the keyboard). Hopefully the accompaniment will play a rhythm but your playing won't trigger any chords. Make sure the Chord Fingering isn't set to Full Keyboard.
Idea 2: Set the Split Point to 036 (C1). Start recording and playing, trying not to use the keys to the left of C1 for your performance. However, as soon as the accompaniment begins to play, try to press the A#-1, A0, and B0 keys at the same time (the lowest black key, plus the A and B keys which are an octave above) to see if that toggles on the "No Chord" option.
Idea 3: Find an external style file that has the rhythm you want to use, and use a third-party utility like MixMaster to remove all of the style parts except the drums and percussion, so you can use the modified style file.