I have designed a circuit that will allow you to connect a Märklin 6017 booster to a Märklin Mobile Station or Central Station, and have the reverse mfx signalling sent back to the Mobile Station or Central Station.
The reverse mfx signalling is based on the principle that the lok decoder modulates the current consumption. These current variations can be picked up from the boostered section, and be used to create a corresponding variation of the current consumption in the track directly connected to the Mobile Station or Central Station. The principle is shown in the figure below.
Figure 1. Booster connection for mfx feedback, principle.
My measurements have shown that the lok decoders transmit the mfx reverse signal with a signal strength of approximately 130 mA. The Mobile Station is capable of detecting a 13 mA signal. I have designed my circuit so that it detects signals down to around 10 mA, i.e., it is slightly more sensitive than the Mobile Station. The transmit part has 7 resistors (R3-R9) in parallel, and the three straps S1-S3 allow you to select the drive strength back to the Mobile Station/Central Station from 20 mA to 140 mA in steps of 20 mA.
I have only tested this with a Mobile Station, since I do not have a Central Station. I do not know if you need the so called "rockers" between the boostered section and the section connected directly to the Mobile Station. In my tests I have kept the two sections separated and I have not tried to run trains between them. The "brown" of the two sections are connected of course.
Schematic
Figure 2. Booster connection for mfx feedback, schematic.