I think post-processing InDesign layouts is generally backwards for good reasons. It may seem like using Photoshop to "grunge up" InDesign text is a good idea.... However, if there's so much text that InDesign is warranted, then there's too much text to be post processing InDesign files - you'll be creating a rabbit hole of problems for later. And, if there's not that much text, you may not need InDesign. All layouts/designs do not have to use InDesign.
The proper way to construct files, if InDesign is used, is to create the imagery you want for the layout and place that into InDesign... if that means rougher, less refined images, that's fine, but they should be processed within the layout, not afterwards.
I've honestly never "post processed" InDesign work. Although, I think one could if it reaches the desired output.
Thinking out loud...
INDD -> PDF -> Linked PS Smart Object would allow the use of INDD for layout and Photoshop for more "textures" or "grain".
Regenerate the PDF when layout changes, Update SO link in PSD which contains any specific post processing there.
Where it gets tricky is if one wishes to narrow the scope of any "unsmooth" alterations in the layout, since the SO would be one layer/object. And of course you'll need one Photoshop document per page/spread which could become cumbersome. So.... then it's back to traditional method of introducing "unsmooth" images in InDesign.
Rougher or less "computer generated" layouts/projects are completely possible, but it all boils down to planning ahead in some way.
- Is the layout very text heavy across multiple pages (books, brochures, manuals)
Then I'd use InDesign and "dirty-up" any placed imagery there.
Rather than merely drawing a frame and setting a fill color. I'd place an image with the color and grain desired. With a large rectangular image from Illustrator/Photoshop, you can fill any sort of frame in InDesign to achieve that color with that grain. Additional images would contain any "roughness". I've often created simple, full page, texture grain or pattern -filled rectangles in AI/PS merely so they could be placed into INDD.
- Does the layout have a moderate amount of text (One pages, trifolds, etc)
In this case, I may choose to use Illustrator for layout. This would allow more direct access to some things of a more "freeform" nature beyond the capabilities of InDesign. In many instances not using InDesign can "free up" the overall appearance of a layout. Breaking away form the seeming perpendicular nature of InDesign layouts is much, much easier in Illustrator.
- Does the layout only contain minimal text (Fliers, Posters, announcements, etc)
In this instance I may just jump to Photoshop and use that. Obviously Photoshop allows much more freeform painting than the other Adobe apps. If there's no need for deep text engine features and I understand output well, I see no reason to not use Photoshop for some more one-page type of projects if a desired art style is more sensibly created in Photoshop. One can always link to text or other items from Illustrator as a smart object if necessary to make editing easier.
In the past I've also...
- Completely hand drawn a layout.
- Manually written/drawn all text.
- Scanned the drawing.
- Broken apart the scan in Photoshop into individual "sections".
- Redrew/rescanned a couple sections I was unhappy with.
- Recreated some parts in Illustrator using the scan as a basis for manual tracing.
- Gathered all the parts together using InDesign as more of a "puzzle frame" to collect all the pieces and position them.
- I saved many pieces as one-color (BMP) .Tif images. So, when placed into InDesign, I could easily assign color there. This allowed for repetitive use of the same art with varying color assignments.
This generated a very freeform style of layout that had a great deal of manual appearance, but offered the benefits of software in terms of being able to individually move or tweak one specific aspect.
I supposed I could have pieced things together in Illustrator or Photoshop, but InDesign seems, to me, to be easier when managing multiple linked files. And ultimately, that's what the layout was.
SIDEBAR: I'm really not a fan of "pasting images into InDesign". That can make for such headaches later. Link to.. don't paste.