It really sounds like Pro Rocker is not easy to come by:
If it were new and there were no parts available, I could see (attached thread) putting in a Kahler, but on older stuff, it's always best to keep it original because it could be worth a lot more someday.
With so many older Ibanezes made with the Pro Rocker, you would think that there would be a lot of them on eBay.
I think the design of the locking nut behind the regular nut (like original Kahlers) was not a great idea, but at the time it was still better than regular Fender trems and Bigsby trems in terms of tuning stability. Eventually as you know it all went to a Floyd style locking nut but that makes the DT355 a collector's item from a certain era with old school locking nut.
If I got this guitar, being an older Ibanez with behind the nut locking nut, I would keep it 100% percent original because many guitars like these were changed over to be routed (by player or luthier) with Floyd style locking nuts. I did this to a guitar that had an old style Kahler and the Floyd locking nut worked way better.
I think without any routing on nut area, Floyd Rose has come up with a stable locking nut that attaches right to the top of the headstock and it only requires a couple of small screw holes.
That's the most I would do (and would do if I had to compare this to any modern double locking trem guitars) and it would make the DT355 much more stable and be a working guitar almost as stable as the Edge.
I am sure there's a nut width and shim that could make the current Gibson style Floyd Rose trem nut work, without digging out half the nut area to make a superstrat Floyd locking nut work.