Soft tread stirrups or EZ rider stirrups, both from www.performance-equestrian.com are very lightweight. However they have a wider tread for endurance, so I don't know if they would be suitable for your purpose or not?
Worked a treat for one girl on the yard - She was making a indians fancy dress costume - The idea of bareback wasnt going to sit with her mother (or the show stewards!)- so brown stirrups were needed!