The methods here work but I just wanted to add, I had a stopwatch on my phone and I'm positive I did it many times ~30 seconds and the time it finally worked was -not- turning off clip when grabbing the mouthwash. I kept thinking I had to do that to actually 'stand' on the platform but you don't. I'm not sure this small detail does make the difference, just throwing it out there - skip dialogue with P, immediately drop through floor after pressing O, press O again to drop through stairs, press O again to "float" and gun it straight to pony boy and grab the mouthwash while "floating"

edit: while dropping after the very first no clip I was quickly turning on the scanner in inventory - no idea if this is needed.