With .999 gold, you can do what you like with it just don't make the mistake of putting gold in an Oak case that is kept in your (unvented) safe with vulnerable items for the long term, this includes silver paper and card. 916 is a little more vulnerable but there would need to be a significant issue to release enough gas to effect such a tiny amount of copper
Having worked in product design and manufacturing for 25 years, I can assure you that outgassing is real. I can also assure you that it is vastly overstated in many cases. Green timbers i.e undried will out gas and ruin silver extremely quickly but I'm yet to meet a furniture maker/cabinet maker working with unseasoned timber. Out of the kiln, timber is circa 10% moisture it is much more likely to draw in air and moisture to equalise with the atmosphere than expel anything. In addition, your coin is in an unvented capsule which will not encourage air to flow into it so compounds must try and pass through it which some can however, that would only occur if the gas had nowhere else to go, like water, it will always take the easiest route.
Personally, I'm much more worried about cheap plastic and an unstable environment. If the humidity in your room varies wildly so will the moisture content in the wood and that gives any chemical compounds an easy ride out of there as it breathes. Poor ventilation will compound that even further. High temperatures and direct sunlight are also no-nos because they tend to excite and expand things.
Buy your box of choice place it in the storage location, opened, for a minimum of 48 hours before placing coins and keep your environment stable. A good rule of thumb (but by no means foolproof) is to use your nose. if you can smell the wood without working it, forget it, no matter what the wood is. Beech at the extreme can appear perfectly dry with almost no taint, it's pretty, quite tough, and cheap, PERFECT for boxes but then you pair the board down and hit a vein of pure sap which is full of sugars, terpenes, tannins, and a bunch of other stuff.
I don't see the problem with an oak holder that is cared for properly and placed adequately. A closed box is a bit more dangerous for obvious reasons but you could line it with other woods in particular the stuff used in humidors. I forget the name, but it's renowned for controlling internal atmospheres.