Watching this tutorial, I thought it would be quite easy to convert i´t in a c++ plugin, and see if I can experiment with it fucntionality wise.

frequency separation only applied to the Value Channel in HSV mode

Its recommended, like shown in the tutorial, to only mask out certain areas of the image (key fed into the mask of the plugin)

The following options are aviable:

Color Space
RGB / HSV when in HSV mode you can select the channel the frequency separation is applied to

High
High Frequency blur, the higher the value the sharper / more details are taking into account

Blend
Multiplies the High Frequency values around the midtone

Low
Low Frequency Blur the amount of bluring for the low frequency

Unfortunately, I still ecountzer random crashes, and I still have no Idea where they come from, sorry.

Here is an example of a video, wich uses frequency separation only on the hue and saturation channel:

Original Clip by videvo

 And a screenshot of the settings in fusion:

UPDATE Version 0.2
I've converted the plugin to a multithreaded version wich should shelve of a few seconds. Also this plugin now features additional outputs for the low and high frequency passes, for futher individual editing. To merge them I've just hacked together a linear blend node, wich does features the same options like the FreqSeq plugin (RGB and HSV blend modes / wich channels to merge.

FS plugin in combination with the channel keyer. Original Clip by videvo