You can make special requests at just about any Disney restaurant. Just call WDW and ask to be transferred to Ohana. You can usually speak directly with the chef. If he/she is unavailable, you can leave a message for them to call you back. I've done this a few times with other restaurants. And they do call you back! It's probably best to wait until a week of so before your PS so your request doesn't get lost. Another option is to visit Ohana a day or two before your PS if you won't be eating there first thing during your trip. Then you can talk to the CMs and chef in person.