Is it possible and safe to use a 5V tolerant output pin with a 100ohm series resistor and a 5k pullup to 5V to drive the signal input of a WS2812, the pin would be in OC output mode ?
This might work.
Then again, simply driving the WS2812B with a 3.3V signal might work too. A few years ago, it happened to work on pretty much all the WS2811 and WS2812s, even though the spec sheet says either 3.5V or 4.0V is the minimum for a guaranteed logic high. Yes, there are multiple datasheets, all in broken English, all of questionable accuracy, some with obvious errors in other places, so the true specs for these LEDs are a matter of some guesswork. About 2 years ago, some WS2812B started appearing on the market with definitely did not work with 3.3V signals while powered with 5V. They look exactly like the ones that can work with 3.3V, so there's really no way to tell other than testing.
So I'd first recommend just trying regular 3.3V output, if for no other reason than learning if your LEDs happen to be ones that can work with 3.3V signals.
The pullup with the pin in open collector mode might work. I'd recommend a 680 ohm or 1K resistor for the pullup, connected directly at the pin and to 5V. The pins are rated for 10 mA, so those resistors should be fine. The trouble with pullups is slow rise time, because the resistor has to charge the capacitance of the pin and wire. The WS2811 protocol depends on pulse width, so different rise and fall times will change the waveform shape, which directly corrupts the signal. Lower value resistors will give you faster rise time.