Broadcast bootloader for up to 150 µCs ?

We are thinking of a machine control system consisting of up to 150 ARM-Cortex-M0 (or M0+)
connected to one RS-485 half-duplex bus (two wires only) @38.400 bit/s.

Upgrading firmware in-the-field sequentially (point-to-point) is working, but would last
several hours, blocking machine (no production). This is not accepted by customers.

So we need a somewhat 'Broadcast Bootloader' with sequential result control afterwards.

Did someone already face a similar problem (or has ideas how to solve) ?
Best regards Juergen

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