The AppX engine is not public like Windows Installer. So there is no point in a custom implementation and additional features are out of the question. Visual Studio 11 offers everything you need to create AppX packages. If you need more than Visual Studio 11 offers, you can't use AppX.
windows8wonder wrote:Doesn't this seem a bit like predatory practice on part of Microsoft, to be pushing long-time install vendors out of "the game" like this?
Not really. Current MSI and EXE installers will still be supported a long time from now. AppX applications have a different target, this is why Microsoft uses a closed system.