With the help of Paul Fertser I was able to install OpenWrt onto my Meraki MR18 access points using a novel appropach of using OpenOCD on a Raspberry Pi. I've had a go at writing up the process here:
https://openwrt.org/toh/meraki/mr18/jtag
Also, I summarised on the existing MR18 page:
https://openwrt.org/toh/meraki/mr18
I have some photographs to add, but at present I "don't have enough rights to upload files". Would these be desirable and, if so, could someone grant me access to upload? Thanks