Can I run two copies of Qimage (one at home and one at work), as I do with Lightroom? Or would I have to purchase two copies to do that? All this would have been extremely easy in Lr, but I need Qimage to overcome the pixel length limit of the driver.
It works exactly the same in LR and QU because you are using the same driver. You need to understand how your printer driver options work so that you can print accurately. Continuing with a 24 x 36 example for ease of demonstration, if you just set your user defined paper size to 24 x 36 inches without changing anything else (from defaults), you
will not get a 36 inch print! Nor will it be 24 inches wide. It will be 23.766 x 35.766 inches because that is the maximum allowable size without borderless. It doesn't matter what software you use: they all use the same driver and all of them will be able to print an absolute maximum of 23.766 x 35.766 inches as that is a physical limitation of the printer (in non-borderless mode).
Here's the rub: Adobe thinks you won't care that your print isn't actually 24 x 36 and there's a 1/4 inch missing from both measurements so if you tell LR (or PhotoShop) to print a 24 x 36, it'll just cut off 1/4 inch without telling you and you'll think you got a full 24 x 36 inch print but it is 1/4 shy in both dimensions. Measure it and you will see. Being actually designed for printing, Qimage is more precise than that and actually lets you know exactly what you are getting so you can't do things like place a 24 x 36 inch print on a setup that Qimage knows is only 23.766 x 34.766 inches.
The only way to fill the 24 inch width is to check the "borderless" checkbox. Continuing with the 24 x 36 example, with Epson, you have two options in borderless (click the "Expansion" button in the driver):
Option 1, Retain Size: your paper size will be 24.233 x 36.0 inches. In this mode, the print is not expanded (stretched) nor is there any overspray, but Epson gives you a ~.12 inch slack on either side of the paper where you can actually print
off the edges of the paper. In this case, when you print two 12 inch wide prints or one 24 x 36, as long as you are using a centered print placement type (like Optimal spaced), it'll center your 24 inch print on the paper and get very close to having the entire image print with no white slivers missing on either edge. They give you the "slack" so you can slide the print one way or another (left or right) to compensate for the paper feeder not loading the paper exactly centered. Keep in mind that when doing borderless printing (trying to print exactly 24 inches wide on a 24 inch roll) you are always going to get a tiny amount of loading "slop" and you'll almost always end up with a tiny white sliver of unprinted area on either the left or right because paper simply cannot be loaded (and stay rolling) that accurately. To prevent this, you can actually print a 24.233 x 36 inch print (use "Fit to Page" size in QU) and that will print a small amount off the edge on each side to prevent the white space. Down side is you lose a tiny amount of the print on both sides because it is actually being printed off the edge of the paper onto the sponge.
Option 2, Auto Expand: In auto expand mode, the print width will actually show 23.997 inches BUT the driver auto-expands the print to be about 24.26 inches. So in this mode, if you want some overspray at the edges to prevent the white slivers, you can "order" or send to the driver a 24 (23.997) inch print and the driver will actually print it at 24.26 inches, overspraying some of the print past the edges.
Personally, I like option 1 because I can control how much overspray I want. I typically use option 1 and select a size of 24.15 inches because that'll give me just a hint more than 1/16 inch overspray on the left/right edges so that I don't get a white sliver down one edge due to paper loading tolerances. With option 1, I can also just expand the width (for example, the 24.15 inches) without expanding the height (leave that at 36.0).
So, knowing how the driver operates should allow you to take the above example and get whatever you like. QU isn't doing anything differently or "mysterious": it is simply allowing you to work on a 1:1 basis with the driver and that's how it should be done because the driver dictates all print limitations and features. The key is understanding how the driver works and what it is doing, particularly if you are printing any combination that requires you to print exactly 24 inches across a physical 24 inch wide roll.
HTH,
Mike