In the end, it shows there really isn’t much difference between the two e200 versions. The Rhapsody models have a modified USB stack somehow that makes it hide the second “hidden” partition in which the bootloader and system software (mi4) is stored.
The Rhapsody bootloader doesn’t allow bootloader updates, and it also actually verifies the digital signature in the mi4 files, so in order to allow Rockbox we have to do a rather funny work-around: use e200tool to make it start the plain e200 bootloader and use that bootloaders recovery mode to upload a binary-patched version of the Rhapsody bootloader. The patched version puts back the old flawed signature check from the vanilla e200 series.
When the old broken BL signature check is in place, we can “upgrade” the target using the normal means and just put a Rockbox bootloader mi4 and the Sansa will then nicely load and run that.