I have an empty slot in the fuse box next to audio. Nothing is labeled on the panel for that slot. It's giving me battery voltage when the car is on. Is it ok if I wire something directly to it with an inline 2 amp fuse (I don't really want to splice into any other wires in the car unless I have to)? Is there potential for damage or something? Also, the bolts down there holding the fuse box seem to be grounded well since my volt meter works, but is that actually a decent place for a permanent ground?

don't see why that would be a problem. just try and keep the fuse as close to the power source as possible. shorts between the fuse and the power source aren't protected. not sure about the ground, but in general look for bare metal, something not painted. you can always ground the device you are trying to power near the device instead of running the ground wire back to the fusebox also.