I think just a resistor is not the right solution for this reason: USBVCC could be powered up by potential state of PIN7, so the current can flow from PIN7 to USBVCC. And the automatic power source selection's circuit could be stressed.
So I think the circuit could be:
USBVCC----------D|------------ PIN7
|
1MR
|
GND
Where:
D| is a diode
1MR is 1MOhm pull-down Resistor
GND is Ground
I can disable internal pull-up resistor with:
pinMode(pin, INPUT); // set pin to input
digitalWrite(pin, LOW); // turn off pullup resistors
Today I will test the solution!