Does
an MPU, e.g. on a Cortex M3, provide enough functionality to support seL4?
No, it doesn’t. The seL4 model is fundamentally based on the ability to virtualise hardware resources.
If you don’t have that, then you can’t dod much more than a memory-protected RTOS. If you’re after an RTOS with a (not yet complete) verification story, look at eChronos: https://ts.data61.csiro.au/projects/TS/echronos/