CL tech support has responded and they've taken the easy way out, basically saying that resolutions above 4K are not supported so we're on our own:

"With reference to your query, as for the current program design, PowerDirector does not support exporting 5.3k resolution video. Manually alter or change PowerDirector's program file or component might have unknown problems while video editing or encoding.

It is suggested not to rewrite or alter the PowerDirector program files to forcefully make PowerDirector produce videos in the not supported resolution.

For your concern, we have escalated the suggestion to support producing 5K/5.3 video resolution to our product team as a feature development reference."

Quote CL tech support has responded and they've taken the easy way out, basically saying that resolutions above 4K are not supported so we're on our own:

"With reference to your query, as for the current program design, PowerDirector does not support exporting 5.3k resolution video. Manually alter or change PowerDirector's program file or component might have unknown problems while video editing or encoding.

It is suggested not to rewrite or alter the PowerDirector program files to forcefully make PowerDirector produce videos in the not supported resolution.

For your concern, we have escalated the suggestion to support producing 5K/5.3 video resolution to our product team as a feature development reference."

Thanks Jeff and Opto - appreciate the responses.