The DiskCacheInspector example program that comes with Xcode will work with the OmniWeb cache. In AppController.m, change "Library/Caches/Safari" to "Library/Caches/com.omnigroup.OmniWeb5/WebKitCache", build & go.
Retrospective is this >< close to being useful here, but it doesn't come with an option to point to the OmniWeb cache.