Section 28.16 Experimentation
Two hooks are provided to allow for arbitrary additions to the LaTeX preamble. It should go without saying that this is for experimenting with new features and is no way supported, including, but not limited to, interactions with current LaTeX packages in use, or those added in the future.
Arbitrary text (LaTeX) used as the values of latex.preamble.early and latex.preamble.late will be added to the very beginning, or very end (respectively), of the generated preamble.