When I can tee up a proof reader .. We tend to export to MS word and then turn on change tracking before handing over the document to the proof readers. This at least makes it easy for us to find the corrections or recomended changes in the word document using its inbuilt change tracking. As the formatting can vary between word and the final help text this is only useful in correcting the text, not finding formatting issues unless they are glaring ones.
Making the corrections back in the help text is still a bit of a pain, sometimes it can be hard finding the relevant words or paragraphs in the help text to make the corrections.
An ideal solution although I assume way too big a project, would be to allow the help text to publish a proof read document in electronic form that is in the format of the final output. Proof readers can edit, make changes or add comment etc. The editors can then review the changes through the help text editor and have change tracking options similar to work like Find Next Change, accept, or manully update it directly in the editor. Basically once the proof readers complete their proof read, you can jump into the Help and Manual project and reconcile the changes directly.
That would be ideal but I imagine a massive undertaking to impliment
But hey, you have to put the ideas out there.