Does Vassal support hidden dice rolls? Take COH for example. Could you roll 3D6 for some hidden action points, then when you expended them all you reveal to your opponent that you did in fact roll x amount?
Not really, but it could be done, I think. I would look into the Layer trait. Use the random layer feature to get three die rolls, reveal them at your leisure.
Yes, it can be done. Just remove any button/tooltip/report text from the die and invoke it via a hotkey from some other piece. Then you are free to query the die result from the corresponding global property (e.g. $result$) and do whatever you want with it, including showing its actual result later on via a mock die piece using layers.
For a quick example you can look at my Marvel Heroes module: “invisible” dice definition towards the bottom, mock “die” in the game piece palette called “chits”.