My limited understanding of ambisonics suggests I could limit the first channel (zeroth order) of an ambisonic signal as necessary and then just make the exact same adjustments to all the other channels? is that correct?
My intuition says that this should work, but I’m not 100% sure. Ambisonics experts may want to chime in.
The IEM plugins have a multi-band compressor that does not alter the spatial image: IEM Plug-in Suite. You can use it with the VSTPlugin extension (VSTPlugin v0.6.0 -- final release!)