What is the easiest (least complicated) way to emulate a "short" button press that pulls a pin low?
This may work:

POSSIBLY a capacitor to ground and a very large value pullup to half supply will work. It may be that an eg 1MOhm to Vin is seen as an open - or a 1M/1M divider so the pin sits at 2.5V or so.
THEN a cap to ground will start the pin at 0V and then charge the cap.
It MAY be that there is an in IC pullup to some OC value so only a cap is needed. 

This will work:

A basic timer can be made with an eg NChannel MOSFET a cap and a resistor. 
I've added a 10k resistor from FET drain to pin 5 due to updated user information. 

On power up cap pulls FET on,
FET grounds pin 5.
Resistor discharges cap and FET turns off.







simulate this circuit &ndash; Schematic created using CircuitLab