Just to document it for others who want to use a different external .fea file for each instance’s contextual kerning:
In the Features tab, create the kern feature, inside :
# Automatic Code End
lookup contextualKerning;
In the instance, create a custom parameter Replace Prefix :
privat;
# Automatic Code End
include(light.fea);
Where light.fea is a .fea file in the same folder as the .glyphs file.
Then the format of the light.fea is as follows:
lookup contextualKerning {
pos A' -500 B' -500 C;
} contextualKerning;