Question: Why ffmpeg plugin make changes in transform step, but not in post?

[Fire7]

First of all, thank you for such a great tool.
We're enjoying it a lot and find it really helpful.

So my question is about the ffmpeg plugin and why it runs at transform step instead of post (like compress do).

Let me give some more context:
In the realm of caching I created a new plugin for unique assets paths, and it overrides output paths for files.
It works great with compress, that makes it stuff at "post" step, so hash in filenames saves for newly created files and it works like a charm.
But with ffmpeg (for audio files) the same trick does not work out of the box, mainly because it write files async at transform step and my custom hash plugin can't manage these files...

So, can you help me with it or maybe fix it? :) 

Thank you.