Would it be possible to include an option to point to an already existing installation of ffmpeg.exe instead of needing to use the one in the install package?
Technically, yes, but there are two issues:
1) Rewriting a portion of the code / interface to check for previous installations and point to them.
2) Ffmpeg is open-source, so there are many different builds floating around. Some builds may not contain some or all of the switches for showspectrumpic. I would have to create debugging for that as well.
I've tried to directly modify and rebuild the Ffmpeg source a couple of times, but haven't had the best results.
In the future, I'll either implement your suggestion or create my own stripped-down build of Ffmpeg for this plugin.
When I do my edits later today/this week, I'll probably take care of this.