We're talking about SS13 here, there are quite a few lists of things that are not practical. I think we could make space for another syndie weapon.
That's one of the few things I hate about the SS13 community, how quickly people are to throw ideas into the ground because it's hard to see them truly existing. Meanwhile you've got an item that can disguise you as a pen, a small handheld device that can be set to 'Airlock', 'Wall' or 'Floor' and within seconds replicate whatever you like AND a singularity that you can visibly see.
Let people have their geeky fantasies damnit, most of them are cool.
I think it's a grand idea if I'm honest with you. Maybe you could have a thin layer of plastic over your lips, with the poison facing outwards? You 'apply' the weapon like an implant to the mouth, and then after using a special verb on someone next to you, you'll poison them.
Of course, I like the idea that they can resist this also, don't want no Rambo Lovers. I like the idea of the poison being kind of lethal, maybe paralyzing if not lethal? Sleep toxin just seems boring when compared to sleepy pens which are far more efficient. If the lipstick is going to have sleep toxin in it, I don't think it should cost as much as a sleepy pen, maybe make it cheaper?
Overall I think this is a bloody good idea and I can see it being used without any problem.