ReplayKit Broadcast Extension: Foreground Privileges Needed for AVAssetWriter

Problem

In the iOS 11 beta, I am successfully receiving video and audio sample buffers in my implementation of RPBroadcastSampleHandler. It's a broadcast extension with an NSExtensionPointIdentifier of com.apple.broadcast-services-upload.

From beta 1 through beta 4, I was able create an AVAssetWriter in this extension. Receive video and audio sample buffers, and write them into a QuickTime movie file.

In beta 5, I'm no longer able to get an AVAssetWriter to work. The call to startWriting() fails with the following error:

Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={
    NSLocalizedFailureReason=An unknown error occurred (-16980),
    NSLocalizedDescription=The operation could not be completed, NSUnderlyingError=0x1c0047590 {
        Error Domain=NSOSStatusErrorDomain Code=-16980 "(null)"
    }
}

Details

I believe I have tracked the issue down to the following log messages in beta 5:

-CMSessionMgr- CMSessionMgrHandleApplicationStateChange: CMSession: Client <extension-bundle-identifier> with pid '<number>' is now Background Running. Background entitlement: NO
...
-CMSessionMgr- _CMSessionBeginInterruption_WithFlags: CMSessionBeginInterruption failed as client has insufficient privileges to take control

This is corresponding log message I used to get in beta 4:

-CMSessionMgr- CMSessionMgrHandleApplicationStateChange: CMSession: Client <extension-bundle-identifier> with pid '<number>' is now Foreground Running. Background entitlement: NO

Prospects

At this point, I can only think of three potential solutions:

1. Figure out what is required to make my broadcast extension run in the foreground.
2. Figure out how to get an AVAssetWriter to work in the background.
3. Hope that Apple increments the privileges of com.apple.broadcast-services-upload from Background back up to Foreground.

Does anyone have advice for any of these? Is there a different solution that I have overlooked?

Thanks,

Otto