I would like to know if it is allowed since it might speed up the development of some apps that have modest UI and OS integration requirements.
Clause 6.3 ("Apps that do not use system provided items, such as buttons and icons, correctly and as described in the Apple Macintosh Human Interface Guidelines will be rejected.") comes closest to not allowing a webapp-wrapped-in-webview, although "do not use [...] correctly" is a vague statement.
I found the Mac App Store requirements in this pastie: http://pastie.org/1236378 via this article (The Register): http://www.theregister.co.uk/2010/11/01/mac_app_store/