I've thought about how cool it would be to have bolt hold open on the PS90 too.
But I can't thing of a way to make it work.
You know most bolt hold open on rifles works by a part that is pushed up by the magazine follower into the path of the closing bolt.
In the case of the PS90 the follower isn't longer than a round- so you can't have a part that is pushed by the follower, and not a round (besides, the follower stops at 45 degrees rotation).
I guess it could be done electronically. A magnet in the follower, a sensor, battery, solenoid, and release button would do the trick.
But that's getting a little crazy.