Using a replacement SPI 16Mb / 128Mbit IC W25Q128FVSSIG you can upgrade the flash in a WR703N / MR3040 / WR720N / MR10U / MR11U / MR13U from 4Mb-8Mb to 16Mb.
The W25Q128FVSSIG is commonly available on aliexpress for a dollar each. And uniqiely, it’s a 200 mil SOP8 not the usual 16 pin that 16 mbyte IC’s are, so it’s pin-for pin with the original SPI IC.. Check W25Q128FV on winbond site.
You need a modified bootloader to recognise it however.(start with https://github.com/pepe2k/u-boot_mod ) as well as a build of openwrt for 16MB flash for your platform.
You can use a dirt-cheap CH341A usb programmer along with a socket for SOP8’s and an adapter board to get data in and out of the SPI to prepare it.
I’ll try and add more detail here about what’s needed, where to get it and how much.
But the most important thing is the SPI programmer.. for less that $50 you’re stuck with CH341A.
And after days of searching I found a driver/software combo that supports 16mbyte SPI.
Programmer – driver – CH341A v1.12
benryanau 
Hi,
great hack!
But you can do “flash burning” more simple: Just use an FTDI cable.
See http://www.tuxad.de/blog/archives/2013/11/24/writing_a_flash_chip_with_a_programming_cable/index.html