I’ve made a few observations. This is brainstorming, feel free to comment.
Observation 1: Since there is no real distinction between tablet and phone apps on Android, I’d suggest that all Android app icons would be generated regardless of any phone/tablet/phone+tablet setting.
Observation 2: There might be some who would still like to have manual control over the iOS icons (ex. different icons for iPhone/iPad versions). A suggestion would be that the presence of a CFBundleIconFiles entry in build.settings would disable auto-generation. This would also have the effect of being backwards-compatible with older projects where auto-generation wasn’t available.
Observation 3: Why not let CoronaLabs lead the way in vector graphics and give us the option to provide an SVG icon file, and have the build script spit out the appropriate PNG files from that.