Lone Wolf Development says:
Setting the visibility of individual items per-player is a priority for us, but the first release of Realm Works won’t provide this ability. The ability to reveal material to individual players is already designed into the infrastructure of how we manage everything internally. However, there's a fair bit of complexity in exposing all the controls cleanly for the GM within the UI, especially when you want to reveal “ItemX” to one subset of players, “ItemY” to a different subset of players, etc. So that's why the feature is not going to be included in V1.0. It should be integrated relatively soon after initial release, since the hardest pieces are already handled.
If they'll put this in, and a built in dice roller to boot, then it's perfect.