Thank you for your answer.
There was a lot of work for me in the meantime. The PC is now at customer site. Thus, my abilities to experiment with patched kernels in knoppix are very limited.
The basic message is that I need to learn more about the difference between software mdraid and BIOS-supported-fake-hardware-but-in-the-end-also-software dmraid. Anyway the whole dmraid stuff seems a little unfinished/ awkward to me.

One general question remains. Is this way of retrieving and putting back images really a gain in failure safty compared to the BIOS-supported-fake-hardware-but-in-the-end-also-software RAID5 with hot spare? At the moment I cannot confidently make this statement to a customer.