NB1. Doing this per note_column would be more complex, but for something like patterns, tracks or instruments it works really well. These lists are more easy to 'mirror'.
NB2. Do note that it's probably not something you want to do per note_column anyway, as custom properties creates some slight extra overhaul (probably due to native __index metatable stuff)
NB3. It's per session only. Any persistent storage would require something like xml dumping or song comment abuse.
Edited by joule, 19 September 2018 - 15:19.