Hi Hannes,
At the moment, there is no support for MIPS CPUs without a floating point unit. Support could be added by following what we already do on the ARM side, though. Once that is in place, I don't believe any further changes would be needed to support soft-float. Since unboxed doubles would be disabled, we wouldn't be making any floating-point calls from generated code.
As for supporting different endianness, I'm less sure of the changes that would be needed. I would proceed by beginning with the simple assembler tests under runtime/vm/assembler_mips_test.cc, which are built into e.g. out/ReleaseXMIPS/run_vm_tests, trying increasingly complex tests from there, and seeing what breaks.
This work would probably take 2+ weeks even for someone already familiar with the code, and at the moment we don't have plans to add support for this configuration.
Even so, let me know if you'd like to try to tackle this, and I can provide more guidance.
Cheers,
Zach