Thank you for the information. It looks like that board can accept serial commands, so if you are using a Mini Maestro, you can send it commands from a script using the SERIAL_SEND_BYTE command. Note that this command is not available on the 6-channel Micro Maestro. If you are planning on using the serial interface, you would connect the TX pin from the Maestro to the RX pin on the DFplayer, the RX pin on the Maestro to the TX pin on the DFplayer, and a ground connection between the two.
Alternatively, it looks like the IO1 and IO2 pins on the DFplayer are intended to be used with buttons to either play the next/previous (with a short press) or increase/decrease volume (with a long press). So, you could probably configure 2 channels on your Maestro as outputs, then connect them to IO1 and IO2 and use them to emulate button presses. Note that in this configuration you would still need a common ground connection between the two boards.
Brandon