Hi,
The way would tackle this (I've done it before) is to make 5 separate images by dividing the image into 5 separate vertical strips.
This can be done in the image editor, You get a read-out of the pixel size as you crop - you need to do some arithmetic first to calculate the pixel width of each strip.
In the editor make the first left had crop taking are to get the exact pixel width. The crop the top & bottom as required. Save As the image naming as, say, crop1.
To make the 2, extend the RH crop line of crop 1 to the right to get a pixel width of 1+2. Now move the LH crop line to the desired pixel width. This method ensure a perfect match between each vertical strip. Adjust the top & bottom crops as required and Save As "Crop2". Do this process for the remaining 3 strips.
When the custom print sizes you'll need to do some more arithmetic to ensure they are all in proportion with each other and equal in proportion to the pixel sizes.
So if the middle image is 500 x 2500 pixels and is a 20" high print, then the smallest image is 500 x 1500 pixels, its print height will be 1500/2500 x 20 = 12".
If the width varies, a similar calculation will be required.
I hope this helps, there may be an easier way, we'll see if anyone else has another suggestion.
Terry

Just to follow up on this: Poster printing is basically for making prints where the page/paper is smaller than the desired print size. QU will automatically make multiple pages with a section of the print on each page.
You could make QU do a poster print by using a custom page size that is say (from your example) 1/5th the with of the total. The height would have to be the same for all 5. EG. Total Print size 30" wide  x 20" high. Set the page size (printable area if sheets) to 6" wide x 20" high. QU should then make 1 row of 5 pages as separate prints.
Terry