Beaky,
Yamahas have a user-adjustable Sync Stop "window," which allows the style to function normally most of the time. But if you release the keys for a pre-specified duration, the style will stop completely. The next time you press a key in the chord area, it will restart the pattern on beat 1. You might have to adjust your playing style a little to compensate (hold your chords down, instead of press and release.)

If it's a slow piece, another option would be to use one of Yamaha's great "free play" styles, which are purposely created to allow playing rubato. I was able to translate one of these to work on my Korg PA800, but they sound better on their native Yamahas. I wish the other manufacturers would develop factory styles like this!

Finally, there are 3rd party tools that allow insertion of a 3/4 fill in a 4/4 style. I've never used it, but the creator of this utility was either Michael Bedesem, Jorgen Sorensen, Paul Tumelty, or Heiko Plate (Forgive me guys, I can't remember which one!) There are tons of 3rd party tools for Yamaha styles that allow a user to do things on a PC that you can't do on the board itself. Good luck!