I don't know how well it would hold up. Keys and change don't do well with car paint.
I'm also not sure about adhesion. At the least, scuff it up with sandpaper and fine steel wool to give the paint something to hold onto. If the case is aluminum, you might need to use a self-etching primer to get the paint to hold.
One option you might consider is having it powdercoated. You migh be able to have it done under $10 or so because it's small. Powdercoat is much more durable than paint, as well. You can DIY powdercoating if you have an old toaster over you don't want to cook in again.