If one wanted to take a complicated HTML document (say the output of the OCaml manual build process) and postprocess it to alter the CSS and some of the HTML, would Lambda Soup be the correct tool for the job?
Yes. I used it myself for post-processing the output of ocamldoc at some point, and IIRC Anton does the same for the online documentation of lambda-soup.
Whether writing a post-processor for automatically generated documentation is a good idea is another topic :-). But in any case, I believe lambda-soup is the right tool for the task.
Sometimes, kludges are expedient. Sadly, for the case I have in mind, postprocessing seems to be the best of bad alternatives.