As I mentioned, I haven't played with item/weapon modification much. If I had to guess, the watcher's blade visual might actually be baked into the model, and the emissive color just sets some element's glow color. That's what I get for just grabbing a random example.
Try this:
"OnEquipVisualEffects": [{ "VisualEffect": "prefabs/effects/abilities/wizzard/fx_citzals_enchanted_armory_sword_wpn.prefab", "AttachPoint": "Fx_Bone_01", "EmissiveColor": { "A": 1, "R": 0.8313726, "G": 0.7764706, "B": 1 }, "EmissiveGlowScale": 1 }],
I slapped that on to the model of the rapier (as well as Modwyr) from your screenshot, and I had a glowing weapons. Technically speaking this is the visual from Citzal's summoned greatsword, but it's pretty close to the visual of the watcher's blade.
You should be able to use any of these attach points. Since some items are using skeleton attach points (RightHand), it's probably safe to say you should be able to attach to any skeleton element.
"AttachPoint": "Fx_Bone_01",
"AttachPoint": "EmissionMesh",
"AttachPoint": "RightWeapon",
"AttachPoint": "RightHand",
Here is a list of some of the Visual Effects from the items.gamedatabundle. Since some of these appear to be ability vfx, it's probably safe to say you should be able to use any of the visual effects from the abilities.gamedatabundle as well.
"VisualEffect": "prefabs/effects/abilities/wizzard/fx_citzals_enchanted_armory_sword_wpn.prefab",
"VisualEffect": "prefabs/effects/abilities/wizzard/fx_citzals_enchanted_armory_morningstar_wpn.prefab",
"VisualEffect": "prefabs/effects/abilities/druid/fx_great_sword_firebrand.prefab",
"VisualEffect": "prefabs/effects/abilities/wizzard/fx_citzals_spirit_lance_weapon.prefab",
"VisualEffect": "prefabs/effects/abilities/priest/fx_spiritual_weapon_berath.prefab",
"VisualEffect": "prefabs/effects/abilities/priest/fx_spiritual_weapon_eothas.prefab",
"VisualEffect": "prefabs/effects/abilities/priest/fx_spiritual_weapon_magran_sword.prefab",
"VisualEffect": "prefabs/effects/abilities/priest/fx_spiritual_weapon_skaen_club.prefab",
"VisualEffect": "prefabs/effects/abilities/priest/fx_spiritual_weapon_skaen_stiletto.prefab",
"VisualEffect": "prefabs/effects/abilities/wizzard/fx_concelhauts_parasitic_staff_wpn.prefab",
"VisualEffect": "prefabs/effects/abilities/wizzard/fx_llengraths_warding_staff_wpn.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_hel_beckoning.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_torchflame_wpn_green_unique.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_torchflame_wpn.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_lantern_flame_wpn.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_candle_flame_prop.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_lethandrias_devotion.prefab",
"VisualEffect": "prefabs/effects/abilities/priest/fx_spiritual_weapon_gaun_lantern.prefab",
"VisualEffect": "prefabs/effects/abilities/chanter/fx_animated_great_sword_summon.prefab",
"VisualEffect": "prefabs/effects/abilities/chanter/fx_animated_pike_summon.prefab",
"VisualEffect": "prefabs/effects/abilities/chanter/fx_animated_wand_summon.prefab",
"VisualEffect": "prefabs/effects/abilities/chanter/fx_animated_war_bow_summon.prefab",
"VisualEffect": "prefabs/effects/abilities/cast/fx_cast_cipher_generic_head.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_engoliero_me_espir_wpn.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_sun_and_moon.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_magistrates_cudgel.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_thundercrack_wpn.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_animancers_sabre_blade_wpn.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_grave_calling.prefab",
"VisualEffect": "prefabs/effects/items/weapons/fx_blackbow_wpn.prefab",