Describe your suggestion...
Add another method to make armor trims emissive, is just add <suffix.emissive value> to the trim palette
e.g,

Every trims:

in assets/minecraft/textures/trims/color_palettes/
after doing this:

every armor trim will be emissive

Selective trims:

in assets/minecraft/textures/trims/color_palettes/
after doing this:

selective armor trim will be emissive
amethyst diamond diamond_darker emerald redstone

Does another mod also do this?
idk

Does OptiFine do this?
no

another way is to add a new property in emissive.properties, it could be named armorTrim.emissive or smth and it would have to form of values. all and <NAME OF MATERIAL>
e.g.

All trims:

in assets/minecraft/optifine/emissive.properties

suffix.emissive=_e
armorTrim.emissive=all

this would make every trim emissive

Selective trims:

in assets/minecraft/optifine/emissive.properties

suffix.emissive=_e
armorTrim.emissive=amethyst diamond diamond_darker emerald redstone

this would make amethyst diamond diamond_darker emerald redstone emissive