By idiomatic I mean using more of the piano's capabilities, like pedaling. Or by using open chords instead of block triads. Or just letting the notes sustain. I realize there is no expression in these types of renderings so I can use my imagination and hear someone who can breathe life into the part. But still, there are things you can do regardless, like playing a bass note, holding it with the pedal and playing less repetitive lines in the right hand. Then you don't have to keep re- striking the note, unless of course you really want to. Basically the part just needs more variety. Open chords are not strictly idiomatic to the piano, as you know. I would use them for strings or brass or anything else.You might also put more melodic material in where the piano is solo. If you do nothing else, just opening up the chords would make a big difference, and your pianist will thank you. I would think it would blend in better with the choir too. Btw, how often are organs available? This would sound nice with an organ.