Ironically in Runescape there ARE trimmable armors, although they're more like decal kits for Dragon Armor, though the kits are from Treasure Trails....i assume they made Kits from Elite TTs because if they added actual Trimmed Dragon to TTs as rewards, then some of the rarer pieces of regular dragon may have fallen in price such as Full Helmets. (Fulls are only dropped by Mithril Dragons, or rarely from cremating chewed bones on a Pyre Ship)
Theres even dragonbone kits that can be used on Dragon and Infinity Robes...and Necromancer kits that can be placed on Dagon'Hai robes...so in a way they're sort of a "trim", though the items with the kits on them are rendered non-tradable...and i think Necromancer Robes doesn't get the kits back when dissembled...unsure of Dragonbone kits though, but i know the regular dragon trimming kits can be dissembled and tradable again.
Theres even dragonbone kits that can be used on Dragon and Infinity Robes...and Necromancer kits that can be placed on Dagon'Hai robes...so in a way they're sort of a "trim", though the items with the kits on them are rendered non-tradable...and i think Necromancer Robes doesn't get the kits back when dissembled...unsure of Dragonbone kits though, but i know the regular dragon trimming kits can be dissembled and tradable again.