Coming back to this, the idea of a custom subclass for holding multi-valued attributes is very much in keeping with the idea introduced by the AttributeDict class I wrote for issue 2065525. The main difference is that I would not provide any alternate implementations for this class other than "regular list". This would let you experiment with your own implementations and if you came up with one that worked for me I could add it in a future version, either as an option or the default.
Coming back to this, the idea of a custom subclass for holding multi-valued attributes is very much in keeping with the idea introduced by the AttributeDict class I wrote for issue 2065525. The main difference is that I would not provide any alternate implementations for this class other than "regular list". This would let you experiment with your own implementations and if you came up with one that worked for me I could add it in a future version, either as an option or the default.