Sure. I have a post.POST.jsp which takes the XML submitted from the client, makes decisions, and checks for errors.
If there are no errors, it generates a PDF using the pdf generator service.
If there are errors, it uses the GuideSubmitServlet to redirect back to the URL the client came from, with the submitted document included with the errors in the redirectParams, so the user can see the data they submitted to give context to the errors.
We want to use a POST rather than a GET, because there are limits to a GET.
This seems a typical case of 'Revalidate on server' and REST submit action POST capability.
In case you don't wish to write validation rules for client as you stated on other thread, that, you have a dedicated server for validations, then in your custom POST action you can create a POST request as we are doing in out of the box REST submit action.