At the moment I only used the DCUTerm tool. By trying to install the required USB driver including within the NetDCUUSBLoader tool, I got a error message about missing digital signature of the driver.
The "Waiting for EBoot, U-Boot, NBoot, M4 Image..." message does appear after pressing 'd' for the serial download.
Afterwards the bootloader image has been downloaded over the "Transmit Binary File" button. During the download process a lot of unexpected character sequences appear in the terminal and it was not possible to save the image by pressing 'f'.
I found a workaround by using the DCUTerm under Linux with wine. It works as expected and for me it is the preferred platform to download a bootloader image.