Hi and welcome to Beoworld! Great equipment that you have there, and I like the photo of the BS9000 setup, it looks great against the brick. Firstly I can discount Setup B:  The Powerlink socket on the BL1612 converter is an input only, not an output.  It is also only used for special setups to provide extra data alongside the Audio Aux link connection. With regard to Setup A:

• Although I don't have a Beocenter 7700, I just had a look at the circuit diagram and user manual and I think that the socket that you are referring to as a 7-pin DIN audio out is actually the 4-pin line in-out connector that could be used to add (for example) an external graphic equaliser.  It's mentioned on page 5 of the user manual and called 'external tone correction'.  It should be fitted with a little white bridging connector/plug in order for the Beocenter to output normally.
• A better fixed line-level output from the Beocenter would be the Tape 2 Out (phono/cinch sockets).
• Given that there can be no datalink from the Beocenter, you could simply connect the BC7700 Tape 2 out to the BS9000's AUX input.  Then you can switch off the Remote control function on the BC7700 (switch on the bottom on the left) and operate it manually (only controlling BS9000 with Beo4).

If you are already using the BS9000's AUX socket for something else, then you need to introduce the BL1612 into the setup.  Connect to the BS9000 by masterlink and then connect the BC7000's Tape 2 Out to the BL1612's AAL socket.  You need to use a lead that connects to pins 3 and 5 of the AAL, as these are the 'video' input.  Again you will need to switch off the BC7700 remote function.  You need to put the BS9000 in A.OPT 2 (see manual) and then you will be able to select the BC7700 as an input by selecting any video source, eg V.MEM. Note:  I have tried to think of a solution that would enable remote control of both devices, but that would require the BS9000 to be set to A.OPT 4 (where it only responds to Beo4 LINK commands).  Unfortunately I don't think this is possible with the BS9000, but other Beoworlders may know otherwise!

A little suplement to Guy's answer: The Beocenter 7700 do not use the same IR control codes as the BS9000 and can therefor not be remote controlled with a BEO4 or Beolink 1000 unless you have a code converter box. If not this might not be that easy to aquire today. So assuming you do not have this code converter you will not have conflicting remote control as the BC7700 will not respond to the BEO4 and has to be operated seperatly or by it's own Beocenter terminal or Master control panel 7700. No matter how if you do the connection via the 1612 or the AUX socket on the BS9000 you should not be worried about simultainiuos audio output as the BS9000 switch away from the input used for the BC7700 when playing CD or Radio.