If you know the name of the hidden parameter by opening the object and looking at the parameter, but but don't want to have to save a copy of it with the parameter unhidden (in case of a Graphisoft part perhaps), simply create a dummy GDL object, manually add a parameter with exactly the same name and of the same type (e.g. length, boolean, angle etc) with a describing name, then save it. In the schedule click 'Additional Parameters' at the bottom, locate your newly created part, and 'add' the required parameter so it will become visible in the fields dialog. When you now open the schedule any parameters with that name, whether hidden or not, should be visible in the schedule.
You can use this dummy object to add as many hidden parameters from any number of objects - just keep adding new parameters and re-saving it.
Hope that helps!
