hehe, there is a LOT of stuff left out in the Beta... like most of the planes and their associated skins.
Even though there will be multiple choices for skins in AH2, they still need to be built into the terrain by the builder (into the .res file). it won't be as simple (and exploitable) as just dropping them into a folder.
When you've made a new skin for AH2, send it in to HTC; and if they like it, they will compile it into the next build.