Hi Scott,
that's great. I must admit I've never tried it in my psr 1500, but I have done it in One Man Band arranger software (uses psr styles).
It's literally to do with the ntt settings. Intro's have chord progressions, so the ntt setting for ( non drum tracks) is Melody.
I assume you'd just do the same for a variation with a chord progression set ntt to melody instead of chord.

If it was me, I'd be inclined to record your progression variation as a midifile. Turn it into it's own style . One of Michael Beddersoms programs Or Jorgens programs may be able to turn it into a style for you. I used to use One Man Band as a style editor. Then use the assembly function to copy the variation into the original style & save as user style.

The less of the style that is exposed to the editor , the better, just in case they alter settings or something.
Manual will be able to advise you on that one.

It's certainly not as straight forward to do as it is in the korg ( korg can load midifile directly back into the style part) but if it's useful to you, it's worth persisting with.

With the korg I could also use one of the fills for this. The korg fills can be 32 bars long also if they're tapped twice , they'll keep looping like a variation.

Anyway, if I can be of any help, let me know. I still have my little psr1500 sitting on the sidelines. It's been a few years since I dabbled with yammie styles . I still sometimes use One Man Band for my psr to korg conversions, so I'm fairly familiar with the style structure.