I tried your endpage and finally got this approach to work. The EndPage puts the reason code 0 or 1 and the page count on the stack (unreliably for some pdfs) so an alternative is the /PageCount variable and I show both. These count the number of pages and are incremented at the end of the page.

You need to know the number of pages to put the overlay on the page before the page is finished. The "finalpage" variable can also be defined on the ghostscript command line instead of in the file. Run ghostscript with both your endpage.ps and DrawingSheet.ps files as arguments. More work would be needed to adapt this to a DrawingSheet.pdf except that should be possible.

See if this works after changing the finalpage number. The table.ps should be placed on top of the original last page before the showpage is executed.

%!
/finalpage 9 def
<<
/EndPage {
       2 ne {
       1 add finalpage eq {(found last page\n)print (Table.ps)run} if
       currentpagedevice /PageCount get ==
       true
       } if
     }
>> setpagedevice

EDIT: The Postscript Language Reference Manual 3 page 176 says this:

The PostScript interpreter maintains an implicit current page that accumulates the marks made by the painting operators. When a program begins, the current page is completely blank. As each painting operator executes, it places marks on the current page. Each new mark completely obscures any marks it may overlay. This method is known as a painting model: no matter what color a mark has—white, black, gray, or color—it is put onto the current page as if it were applied with opaque paint. Once the page has been completely composed, invoking the showpage operator renders the accumulated marks on the output media and then clears the page to white again.

After many trials and errors and of course suggestions/comments from fellow SO users like @beginner6789 and @luserdroog (my thanks to both of them), I finally got something that works for my case. I am going to summarize my experiments and findings for benefits of others.

For this discussion, let's say you want to superimpose a page from Overlay.* (.ps or .pdf) on top of one particular page from Base.* (.ps or .pdf). There does not appear to be one solution fit all scenarios. It depends on:

• If the you have Overlay.ps or Overlay.pdf. Although converting them is quite easy.
• If the you have Base.ps or Base.pdf. Although converting them is quite easy.
• If the Overlay.ps or Base.ps is auto-generated from corresponding pdf file or if they are handwritten. I found that the handwritten PostScript files are much cleaner, precise, and optimized compared to auto-generated PostScript files.
• Logic becomes more complex depending upon which page/s of Overlay file you want super imposed on top of which page/s of Base file. You may have to do some extra work to identify those pages.

Following are 3 solutions that I tried. To summarize: Solution-3 worked for me as described by the last row of the corresponding results table.

Solution-1:

Create following Merge.ps and run it through your GhostScript to generate PDF output:

1 dict begin
    /showpage {} def
    (Base.ps) run     % or Base.pdf
    (Overlay.ps) run  % or Overlay.pdf
end
showpage

Results:

Does not appear to be a suitable option if Base file is multi-page file.

Solution-2:

Create following Merge.ps and run it through your GhostScript along with Base file to generate PDF output:

<<
    /EndPage
    {
        2 eq
        {
            pop false
        }
        {
            1 dict begin
                /showpage {} def
                (Overlay.ps) run  % or Overlay.pdf
            end
            pop true
        } ifelse
    } bind
>> setpagedevice

Additional logic is required if you want Overlay on only certain pages of the Base file.

Results:

Solution-3:

Edit the Base.ps file that was converted from Base.pdf. Find the entry %%EndPageSetup corresponding to the page/s that you want overlay, and insert following code after that. Run it through your GhostScript to generate PDF output. In the example below I am editing it for the third page:

% BUNCH OF CODE NOT SHOWN HERE %
%%Page: 3 3
%%PageBoundingBox: 0 0 792 612
%%BeginPageSetup
8 0 obj
<</Type/Page/MediaBox [0 0 792 612]
/Parent 3 0 R
/Resources<</ProcSet[/PDF]
>>
/Contents 9 0 R
/CropBox 
[0 0 792.0 612.0]
>>
endobj
%%EndPageSetup
    1 dict begin                    % <== My Edits
        /showpage {} def            % <== My Edits
        (Overlay.ps) run            % <== My Edits
        % [1 0 0 -1 0 612] concat   % <== My Edits. See Notes below.
    end                             % <== My Edits
9 0 obj
% BUNCH OF CODE NOT SHOWN HERE %

Results:

In my case:

• I had no control over how the Base.pdf file was generated, but I could convert it to Base.ps using GhostScript.
• I had partial control over Overlay file generation. I had XML data, which I converted to table format using XSLFO, and then converted to PDF and PostScript file using Apache FOP.
• I really had no option of manually writing Base.ps or Overlay.ps.

Considering, results of all these trials and errors, at this time the Solution-3 (with transformation matrix concatenated after the run command, the last row from that results table) is working for me. I don't particularly like this option, because it involves editing the auto-generated PostScript file.

NOTE: For all those cases where I saw that the page from Base file is being replaced by Overlay page, I think the original page from the Base file is being printed outside the output page size hence it is clipped and does not show and appears as being replaced. Maybe proper manipulation of the transformation matrix would fix those cases. I am yet to experiment with that scenario but don't know how to verify.