Well, there are a lot of places where you'll find how to disable the seat belt buzzer for key start mark x (including pw), but you wouldnt be able to find any where how to do it for a push start.
With some experimentation and some help from lexus is 250 forums, I managed to carry out the procedure successfully on my push start mark x.
Please note that I NEVER drive without my seat belt, nor my family for that matter. We all always wear seat belts and I encourage everyone to ALWAYS drive with seat belts on, no matter even if you have to go to the next block from your house in your car, but I must admit that the seat belt buzzer can be annoying and un-needed some times...like when you're driving off / on / backing into a long drive way, you most probably dont have / dont need your seat belt on at that time..and when the buzzer starts beeping, its really annoying...and there are other occasions too..like if you're keeping a heavy box / pet / etc on the passenger seat, the seat belt buzzer would go off, etc, and thats why there's a reason why theres an option in your car with which you can switch the buzzer on / off, without cutting any wires or making any other modifications.
So here's the guide (tested by me successfully on my own push start mark x)...and this will only disable the buzzer and not the seat belts' flashing lights:
Sit in your mark x (alone, and with no one on passenger seat), and close the door and wear your seat belt. (Yes you read it right...you have to wear your seat belt in the first step in order to successfully disable the buzzer :D).
Press the START button twice (without foot brake) to power up the system.
Make sure the ODO meter is displayed...if its not, then Press the trip reset button until the ODO is displayed (not trip A or B).
Turn off the system again by pressing START again.
Unbuckle your (driver's side) seatbelt.
Turn on the system again (press the START button twice without foot brake).
When the dash lights start lighting up, press and hold the trip reset button for about 15-20 seconds (try for longer time if it doesnt work out).
While still holding the trip reset button, buckle your (driver's side) seatbelt.
Now you should be able to see "b ON" or "b OFF" on your odometer display. Toggle this with the trip reset button until the display shows "b OFF".
Now you can unbuckle your seat belt and wait for the ODO meter to go back to its previous status, and that means that the process is complete.
The buzzer will now remain off, with only the lights flashing. You can switch the buzzer on again using the same steps.
REMEMBER: ALWAYS BUCKLE UP WHEN DRIVING.
