I contacted Hawaiian Airlines both by email, and I spoke a supervisor, and no, they will not be accepting soft bike cases. In both cases the answer i got was no.
Most of the experience from locals has been that if you call the office, they will tell you that it is a strict no soft case policy. But people who have recently asked the airline employees at the check in counter when you show up to the airport all say that they have no idea the policy changed. A friend of mine took his soft case and taped a bunch of cardboard around it last weekend to fly interisland. When he checked in, the airline people were like...WTF? He got too and from Kauai/Honolulu no problem through Hawaiian and I think he didn't bother with taping the cardboard on the way back.
Don't take this as proof that the policy isn't enforced...just that it is *my belief* that this is Hawaiian's way of instantly throwing out any soft case damage claims. They probably talked to a lawyer that advised them if they have a no soft case policy in writing but still let you check in your soft case that they are not liable. As compared to the effort for each customer with a soft case to sign a waiver...because you would depend on the check in staff to enforce it. If they forget, or lose the waiver...then you can claim damages. And as already mentioned, if you are on connecting flights from different airlines, Hawaiian can depend on someone like United to get their waiver signed when you check in. Again...this is only my opinion.
why would you fly hawaiian and take your soft case if this is what you're hanging your hat on?
If every airline paid for damages to your bike in transit...no questions asked...then I could see why you would avoid Hawaiian with your soft case. But the fact is that damages to your bike while in airline transit is for the most part a "shit out of luck" battle regardless of the airline or their bike case policy. They often blame you for packing, TSA, or that your bike was already damaged. It's very hard to "prove" the airline at fault unless you are ultra vigilant. So if you already invested $500-600 in a really good soft case which is 90% as protective as a hard case (read...not a scion case), then you may hang your hat on Hawaiian if indeed they will let you check it in but throw out all soft case damage claims immediately. For locals who have Pukalani status and use the Hawaiian Airlines credit card for miles...Hawaiian is often best option ...especially interisland.
Furthermore, most instances of damage come from TSA opening your case, moving things around and not putting the case back together properly. IMHO, this happens most often with hard clam shell cases because it's possible to just force the thing shut with straps. I have this type of hard case and even for me sometimes I have to take a few extra minutes to shift things around to make sure the case closes properly without anything being smashed. With something like the Thule Pro XT, the zippers make it easy for TSA to open up the case while your frame is locked into the bottom of the case. Then they just zip it back up.
If you ask me, I rather have something like a Thule Pro XT case and a "we'll accept it but no damage claim policy" than my current hard clam shell case that has a "you can claim damages but you'll most likely get two middle fingers" policy. YMMV.