Modifying Multiblock Controller UI

Adding text component

To add text component to the UI, you need to use .additionalDisplay in the multiblock registration builder.
.additionalDisplay takes a lambda that takes 2 arguments: the IMultiController machine that the components are being added to, and the List<Component> of existing components.
An example of using it would be:

ui_modified_multiblock.js

GTCEuStartupEvents.registry('gtceu:machine', event => {
    event.create('ui_modified_multiblock', 'multiblock')
        .rotationState(RotationState.NON_Y_AXIS)
        .recipeType('electrolyzer')
        .recipeModifiers([GTRecipeModifiers.OC_NON_PERFECT_SUBTICK])
        .appearanceBlock(() => Block.getBlock("gtceu:solid_machine_casing"))
        .pattern(definition => FactoryBlockPattern.start()
            .aisle('###','   ','###')
            .aisle('###',' S ','###')
            .aisle('#C#','   ','###')
            .where('C', Predicates.controller(Predicates.blocks(definition.get())))
            .where('#', Predicates.blocks("gtceu:solid_machine_casing")
                .or(Predicates.abilities(PartAbility.IMPORT_ITEMS).setPreviewCount(1))
                .or(Predicates.abilities(PartAbility.EXPORT_ITEMS).setPreviewCount(1))
                .or(Predicates.abilities(PartAbility.INPUT_ENERGY).setMaxGlobalLimited(1).setPreviewCount(1)))
            .where('S', Predicates.blocks("gtceu:steel_machine_casing"))
            .where(' ', Predicates.any())
            .build())
        .workableCasingModel("gtceu:block/casings/solid/machine_casing_solid_steel", "gtceu:block/multiblock/blast_furnace")
        .additionalDisplay((machine, components) => { // (3)
            if (machine.isFormed()) { // (1)
                components.add(Component.literal("I am text component #1")) // (2)
                components.add(Component.literal("I am text component #2"))
            }
        })
});

1. Check if multiblock is formed
2. To add new line - Use components.add(Component).
3. Using .additionalDisplay() to add text component.