Can you help me getting started with a DirectShow filter? You're the expert of that.
I don't really know much about DirectShow really, aside from it being based on COM and having a reputation for having a complicated API that's difficult to work with.
Have you considered contributing to ffmpeg, such as adding a new filter to libavfilter? For example the speaker layout could be saved to a file in some format, and then ffmpeg can use that file to render and mix the objects. This would have the most impact as your work would naturally percolate down to any software that uses ffmpeg internally, including LAV Audio and mpv for example. This might also provide some momentum and enthusiasm for ffmpeg reverse engineering specialists to look into reverse engineering audio objects in TrueHD/MLP.
There is a funny addition to this question. When you decode the PCM data of Dolby test tones, the levels are off, one of the surrounds is mixed at a seemingly random gain. Applying Atmos metadata fixes this, the test tones are the same level after remixing it from 5.1 to 5.1.
Given that it's a test file, do you think that might have been done on purpose, to make it obvious that the object layer is not being processed?