This doesn't work:
addUserActionEventHandler ["ReloadMagazine", "Activate", { createDialog "RscDisplayEmpty"; systemChat str inputAction "ReloadMagazine"; //returns 0, other actions don't work either while dialog is open }];
If you remove the createDialog line or replace it with findDisplay 46 createDisplay "RscDisplayEmpty" it works:
addUserActionEventHandler ["ReloadMagazine", "Activate", { systemChat str inputAction "ReloadMagazine"; }];
addUserActionEventHandler ["ReloadMagazine", "Activate", { findDisplay 46 createDisplay "RscDisplayEmpty"; systemChat str inputAction "ReloadMagazine"; }];