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Maybe use anonymous blocks as non-editable (without your application) custom entities?
There are some work arounds but it takes some effort. If I see Preston's webcast then it is possible to draw a line and present that simple line like a thermometer with grips to alter the heat. That is one example. The values kan be stored in the extension database or xdata. The visibility is done by the dll. So if someone does not have the dll, they only see a simple line.
What we can do is create a screen wide text saying there are custom objects in the drawing. The text is invisible if the dll is loaded.
Another solution is indeed using (anonimous) blocks which have the duplicate objects of the custom objects and are invisible if the dll is loaded. If the dll is not loaded, the custom object is not visible but the block is. Maybe... that could be the solution