The imagewriter 15 was model A9M0305 and from what I can tell from Apple's imagewriter schematic, it's the same motherboard, just has a jumper that grounds the 8085's SID input.

It reads that input when it starts up, I just made a config option previously so I could set it to 1 or 0, but now I've got this in:

//    i8085 Main cpu in sid function

uint8_t apple_imagewriter_printer_device::maincpu_in_sid_func()
	return (PAPER_WIDTH_INCHES == 15.0) ? 0 : 1;  // for imagewriter15, will be 0, regular imagewriter will be 1
//	return ioportsaferead("WIDTH");