Printing with my Z3100 I have a similar issue with the printer laying down ink on "white" borders, under one circumstance, and I guess there is no way around it. I have found it economical to print 11x17's 2-up on a 17x24 page and then cut them apart. I use Qimage to print from and I place the 11x17's on the page using the "Optimum" setting, to minimize the cuts I have to make.
Because Qimage will not add unequal borders around the actual print area I add white canvas around the image, bringing the over all size to 11x17. I stroke a 1-pixel line around the border. Using HP Premium ID Satin's profile when softproofing, the images would print with a blue color cast, so I compensate with adjustment layers. No matter how I do it (whether I softproof before the white borders are added or after) the white borders end up less than pure white, and get printed on, even if only the gloss enhancer.
If someone has a solution to this let us know. Otherwise, I willingly make the tradeoff since it is still easier and less expensive than using a smaller printer or single 11x17 sheets in the Z3100. No such thing as a free lunch, I guess.