If you're afraid of not being able to retrieve it, then don't use a very tiny bottle. Those chapstick things might be too small, for this, I would imagine. I had a glass bottle of lipgloss once that (if I still had it) would have worked perfectly. It was maybe 3-4 inches long, and very smooth.
If it did slip past your reach, the key would then be to get yourself "turned-on". So much so, in fact, that your pussy becomes slick with your own juices. Interestingly, your body has a tendancy to push things out by itself. If you try to leave a dildo in your pussy, your body will shove it out, if you're wet enough. I wouldn't worry about not being able to get it. (granted, this method may take some time for it to slip out though, depending on how you're able to tighten/relax your own muscles there, and how slippery you become).
I think dares like this are easy enough to do. I like it. Alternatively, you could stay in the bathroom and give yourself an orgasm with the lip gloss bottle, apply the gloss to your lips, slide it back inside and carry on with your day.
Nice one, Janey.