After looking through some of Rho's own native extensions and perusing their source and rake files, I have discovered a supported (but apparently not documented) way to accomplish this.
In the ext.yml file under <Your extension>/ext add the following line under android: "adds: ext/platform/android/adds".
The "adds" folder can be any folder name you wish, just update the line in ext.yml accordingly.
Anything placed in the adds folder should follow the file structure you would expect in a standard android .apk (assets folder, libs folder, res folder, etc). Any files placed in any of these folders will be copied to the appropriate folder in the final APK.
***One word of caution: If you are using lib files, the Android build does not seem to play nice with multiple architectures. I had folders for armeabi, mips, x86 and all contents of all folders were copied to the armeabi folder in the final apk.
Hope this helps someone in the future.