@sumo that is a strange error. The image is my personal one (with a ton of stuff added) used on two devices here.
A few issues left to resolve on these, I'll give some details:
The kernel is limited to 3MB in this current setup due to the partition layout, using one of the bootkernel partitions. On the MR32 (which has a similar layout) the OEM bootkernel is kept. This kexecs part.safe, so this part.safe can be used to house the OpenWrt kernel. However there are issues when doing this on the MX64/MX65. Firstly the MX64/65 bootkernel resets the coherency regs(unique to this bcm5862x platform), which makes most things stop working unless the 'dma-coherent' property is removed in the device tree (in theory this should affect performance as well). Secondly the i2c bus stops working, needed for the at24 and the PoE.
It was suggested to change u-boot and directly boot an ubi stored fit image. This might be an option however u-boot partition is also restricted to 512KiB, so adding too many extra features makes the image too large. However we can maybe move shmoo, nvram etc to the end (there is 1MB unused at the end of the partition layout iirc). There are a wide range of possibilities here but not sure what is best. These will all make reverting to Meraki OS very difficult. I'm not sure how long it will be before the kernel exceeds 3MB (currently we are at about 2.7).
The second issue is this PoE fw image/driver. The fw image needs to be upstreamed to linux-firmware. In theory only Broadcom should do this. If someone were to try this, showing that the image is extracted from a GPL archive provided by cisco, I am unsure what the response would be. However we can still proceed to add the device without PoE support and add this in later once resolved.