Because that's totally moral and ethical? No conflict of interest at all. What a neat hunter trick. Also, tell them you're their fairy auntie, that'll disarm the marks even more.
The way I see it, and I'm open to being wrong about this, is that people that join a bootcamp are doing so to find a job. If at the end of the course I shoulder tap the best students I've had and we connect on linkedin (or they make the request) and then we end up entering into some kind of internship / jr position, I can only see that as a good thing.
The dev gets a job, I get good staff, the course gets their vague job placement goals met and at the end of the day, software gets written.
The only people I see being disadvantaged is others in the market for a dev. But that's not going to keep me up at night.
In the time you have available, under the conditions you have the candidates, I don't think I would be able to determine competency or even personality traits.
In a 4 week segment that I taught I would start to form opinions of students after ~6 hours of interaction with them. Sometimes arrogance was just nerves, expectations were super malleable and not necessarily deeply embedded.
Competence is almost completely separated from course completion / rate of completion. Effort applied isn't a great indicator either (although slightly better than the other two).
I would recommend if you want to find great candidates, teach a bootcamp. There's literally the top 10% going through that can just be picked off. I hired one of the 3 brightest I taught and the other two I primed for when I have hiring budget.