I did a standard decoder installation in one. The mini plugs might be useful, but I've found that it's easier to just hardwire it all together. One option might be to run the 4 wires you need for pick up and motor power through to a plug inside the tender. Hanging the plugs between the tender and engine will be impractical and bulky.
There's plenty of room in the tender for the sound system, you'll need to do some sort of trick like drilling holes under the coal load casting for the speaker.
Lee