Looked into it, looks like metasm supports a couple different families of mips:

https://github.com/jjyg/metasm/blob/master/metasm/cpu/mips/opcodes.rb

You could try generating an exe with metasm mips CPU with :family => :mips32. Then see if it will run on your device. The default seems to be mipsr32. Here is the test script i was using:

https://gist.github.com/jvennix-r7/53fba81513da75da7cc9

(mostly copied from metasm/samples/exeencode.rb). Hopefully that helps