Andy,
At Bike Farm we have a similar policy. Volunteering for a steady two months can lead to a key. You have to be nominated, and it's the responsibility of the person who nominated you to ensure that you understand the rules of the shop.
We've had a number houseless folks be key holders. Besides the occasional person sleeping at the shop when it's pouring freezing rain outside, we haven't had any issues. But we are still selective of who we nominate.
For example, there's a fellow who's been volunteering for months now. He's super dedicated. But honestly, he displays some paranoid tendencies, and sometimes get's into verbal altercations with others. We vote on who gets a key, and just like all our decisions, anyone can veto a proposal. I would vote "no" if this person came up for key nomination. I have no problems with letting people know why I don't think it would be appropriate.
There are a ton of valid reasons to object to someone's nomination. I hope you and other volunteers feel comfortable enough to express yourselves. At the same time, there are no rules for those valid reasons. Economic status and or past drug use, to me, are not valid reasons to deny someone a key. Try to base your conclusions on their present behaviour, not your assumptions.
It's lucky you get 4 months of time to observe the person in the shop. If after 4 months, you're still not sure if you can trust them, hang out with them more. Try being friends. What's their life like?
It seems like you're searching for a rule to follow, but the reality is that trust is something we develop. Trust is vital for shops like ours to operate.
I think if you really got to know the person, you would feel more comfortable with trusting them with a key to the shop. You shouldn't be afraid to express if you don't trust someone. But one would hope that feeling of untrust is based on their actions, and not stereotypes.
Good luck! Being accepting and conscientious isn't easy work.
-momoko