I think I know the answer, but I want to be absolutely sure and have the answer out there to show my GM in case he disagrees with me:
When you use Versatile Performer (or the Dirge Bard's Haunting Refrain) to roll an instrument-based Perform check in place of some other skill, do you need to actually play the instrument in question?
If I use Haunting Refrain to demoralize someone, do I need to be sitting at a piano to roll Perform(Keyboards) instead of Intimidate?
No. You use the bonuses associated with that perform skill in place of the bonuses usually associated with that skill, but you're not actually using that perform skill. It would make for a pretty crippling tell if every lie you told was in song.