I haven’t had much luck with stability of openwrt on MIPS (might sound vague but i’ve tried many different routeros versions including stable and betas, many different images downloaded from wiki and forums here, always ended up with something wrong). I’d like to try DebWRT or something else as a last resort.
there where some serious problems with metarouter on mipsbe, most of them are resolved in latest RouterOS releases.
But you can make any distro run-able as metarouer guest as long as it is built similar to OpenWRT and has kernel patch. It is just that OpenWRT is already pre-configured to compile for metarouter.