Currently this can be achieved for a field on the same screen only via customization of the velocity templates being used to render the screen. A feature to make this easier is planned for a future version.
For "mandatory" the workaround for this is to implement an Error event rule so that if the user doesn't answer they get an error message. Obviously this has the usual limitations of using event rules that the value will have already been submitted to the rules engine.
You could similarly "workaround" for read-only by forcing the user to not provide an answer (ie the value would be uncertain) - but you'd need to make sure your rules very carefully handled that uncertain and that you didn't need that uncertain for anything else.