Hi,
I've booted up Ubuntu and did a make on the OpenWRT buildroot with the mdadm source included.
No problems cross compiling mdadm 3.2 source for the armv4 machine architecture.
It also created a firmware compatible IPK package for it.

(Isn't OpenWRT wonderfull!)
I'ts about 250 kB in size, and might be too big to fit in the remaining empty partition space of the MBox internal flash memory.
If it doesn't fit, you can delete some unwanted components, or boot into a bigger root file system (eg on a USB dongle).
Hopefully you know your way around Linux better than me, being a mere MS windows user except for fiddling with the NAS firmware.
Do you still want to try it and write a small how to article in exchange?
AQUAR