Hi Tyler,
Are you sure that you are thinking things through clearly here? You are really not interested in DPI at any point through pre- and post-processing. You are only interested in DPI once you have your final image, and have selected from that image the area that you wish to see printed out.
Now, at that point - sure - your image may well be in XISF format, but is it XISF data that you will be sending to the printer? Almost certainly not. You will either be converting the image to another format - one that can be sent to an external print house for example - or you will be using PixInsight to render the image directly to your own print driver, assuming that you have in-house capabilities to generate the size and quality of prints required by your clients.
In either case the DPI need not be stored within the XISF file. It 'could' be defined during either of the two steps I have mentioned above, although it is probably more 'normal' to define a 'physical' print area (in 'x by y' mm or inches) and then examine the returned DPI of the image as it will be printed.
I always use a local 'warehouse store' to print my large-format images. They use photo [rpcesses to create the images on 36" x 24" (914mm x 614mm) board-stock, and this costs around the $10.00 mark. I couldn't even purchase the consumables for that price, far less maintain the printer in the 'tip-top' condition that commercial customers would expect.
And, when I take an image to them for print, I just take the 'final image' to them in un-compressed (or very slightly compressed) JPG format. Of course, I will have looked at the pixel count based on the size of that final image, and would not bother to seek their expeertise if the image to be printed did not have enough resolution to warrant such large-format printout. In other words, I would not waste my time and money to print out an image that was much less than 11,000 x 7,500 pixels. And, to me, that is a really big image to be trying to process in PixInsight - it would be a 3x3 mosaic from my 10MP CCD Imager, for which might have to wait several years to acquire the raw data. (Thankfully, I do not image for anyone, bar myself, and I don't have the patience - or remaining life expectancy - to wait that long for any project to reach completion).
However, maybe I'm missing the point you are trying to make - can you rephrase the question and we can perhaps rethink a solution?
