Currently there are issues with AU and the _ng client, but nothing Face can't solve apparently. But making it _ng compatible it's not a priority atm. But Face can give you a better perspective on that stuff.
IIRC, the shadow problem in D3D9 comes from orbiter_ng.exe not respecting UNDERSHADOW flags. IMHO, the orbiter_ng clients (together with orbiter_ng) should work towards displaying addons like in the inline client. Making addons work for a specific OVP client that is still in development is a somehow reversed approach to that.
There are several problems with AU in OVP clients ATM, the shadows being the least of it. One major bummer is transparency. It just doesn't work in D3D9 or D3D11, giving the base a completely crippled look in OVP. Another one is texture troubles, causing certain texture resolutions to be displayed in freaky colours on my rig.
Of course those issues should be addressed, just not in the AU project, but in the OVP project(s). Therefore my priority as AU coder is on features and not OVP compatibility.
regards,
Face