Hmmm.
It seems to me like you have exhausted all “normal” ways.
From the error you get it sounds like the board is not sending the whatever allows netinstall to select the “appropriate” boot image.
Semi-random thought (and YMMGV) could it be that it is possible to “force” a single boot image (the mips one in your case) with some dnsmasq trickery under Linux?
The method of extracting boot images and serve them via dnsmasq is detailed in this project:
http://forum.mikrotik.com/t/pynetinstall-free-and-open-source-netinstall-implementation-for-flashing-mikrotik-routerboards/166439/1
https://github.com/dvtirol/pynetinstall
No idea if it can work, and it is seemingly some work to setup such a system, so maybe it is better to wait if some other forum member can provide other ideas.