Try spraying or painting the surface with a hard finish material
like shellac, acrylic, or ? that will render the article to be molded
clean and smooth enough to get mold material on it, and not have it
embed, Several layers may have to be applied to seal it well enough,
Just follow instruction on how long to dry between layers, Of course
a bit of detail would be lost,, but think about it,,, the porosity
that would include any undercuts would mar the surface of the mold.
Sprays can fill those places. Care must be taken to apply only that
amount that would prevent mold material (or investment) to get stuck.
The idea about using latex in layers is a good one, but I would also
use the spray before doing that method
This can also be used for burnout skipping the mold step, But I sure
like the idea of having a mold of a great thing!