In Minecraft Bedrock 1.19.81 adding the component:
"tag:minecraft:trimmable_armors":{}

Will allow you to use armor trims on custom armor. however this will only work if you are the world host
*(as of 1.20 you can still trim the armor but the template and material is consumed but no trim is given)
*
(1.19.81-1.19.83)
World host:
When trying to apply a any armor trim to a custom armor it will work as expected (shown in "1.19.81 localplayer" attachment)

Player that joins world
When trying to apply any armor trim to a custom armor it will display:
"Item can't be upgraded this way" (shown in "1.19.81 serverplayer" attachment)

Minecraft Preview (1.20.0.23 - 1.20.10.21) and Minecraft 1.20
When trying to apply any armor trim to a custom armor it will let you, but it will not apply consuming the trim and any material provided (shown in "Preview test" attachment