I tried your idea and wanted to offer and few observations. Making the page size bigger and then repositioning the model works, but it has a few drawbacks. I doubled the size of the page, and then I also changed all the margins so that the PLI and step numbers would still appear near the corner of the real final page. It is difficult to know exactly how far to zoom the LPub window to approximate what the final page will look like, so it involves some iteration.
I got this all to work and then exported the pages and used Acrobat to crop. The first thing I noticed was that my 900 page, 180Mb was the same size after cropping as before cropping. I also noticed that Acrobat managed to crop 900 pages in less than a second. This made me realize that Acrobat does not really crop the images, it only reduces what is displayed. This means that the data in the margins is still there in the file, you just can't see it. So I went back and cropped them all with an action in Photoshop, and then the file got much smaller.
So in the end, this is a viable workaround but it adds a lot of extra labor to your instruction project.
I got this all to work and then exported the pages and used Acrobat to crop. The first thing I noticed was that my 900 page, 180Mb was the same size after cropping as before cropping. I also noticed that Acrobat managed to crop 900 pages in less than a second. This made me realize that Acrobat does not really crop the images, it only reduces what is displayed. This means that the data in the margins is still there in the file, you just can't see it. So I went back and cropped them all with an action in Photoshop, and then the file got much smaller.
So in the end, this is a viable workaround but it adds a lot of extra labor to your instruction project.