While it’s not a good idea to have an unclaimed device for the pick of anybody passing, you can do that.
You’d just need to set the “SETUP_DONE” bit (see code below) and provide the WiFi credentials.
#include "Particle.h"
#include "dct.h"
SYSTEM_MODE(SEMI_AUTOMATIC);
void setup() {
const uint8_t val = 0x01;
dct_write_app_data(&val, DCT_SETUP_DONE_OFFSET, 1);
pinMode(D7, OUTPUT);
digitalWrite(D7, HIGH);
Particle.connect();
}
Once that’s done you can flash whatever code you want.