I realize that you wish to keep your external switching circuit simple, but...
My experience with the external switching on many different MP3 players is that they are extremely sensitive to bounce. Such as what you'd get from a reed switch or any other mechanical switch. The bounce almost always caused the player(s) to hang.
And alec_t is correct, there is a current (very small) flowing in the MP3 switch(s). It is nothing more that an input to the MP3 uC chip that is, ordinarily, held in a HIGH state (as are the other control buttons). But this can be used to your advantage.
The only method for an external switching device, such as you propose, that I got to repeatedly work correctly, was a mechanical switch used to trigger a dual 555 cascade timer that then drove a simple 2n2222 transistor switch with its collector-emitter tied across the MP3 switch (thus isolating the MP3 from the 555 ckt. power): a bounce-less, finger press emulator, if you will.
You have to get rid of the bounce.