Reading through my own post below I realized, if I could flash the firmware named
e200v2 maybe I could also create a boot loader based on the bootloader for e200v2 'bootloader-e200v2.sansa'. I gave it a try and it worked. mkamsboot created a patched boot loader and I could run it on the device.
So as a next step I then used the graphical RockboxUtility-v1.5.1 again, but this time I manually selected 'Sandisk Sansa e200v2 series (Stable)' as the device. I ignored the error message, started the installation, selected the same firmware 'e200v203.01.16e/e200pe.bin' and watched the installation proceed and finish like a charm.
Now I'm unsure if I own a 'e200v2' or 'e280v2' aka 'Sansa Fuze v2' but at the end I don't care about the name as long as it works. For reference I include a picture of the device with Rockbox running on it.
thanks for your hint @bahus