Hi @datamilch,
you can use the EVMSG_CHANGE message as a trigger to check your objects for changes. The timer approach might seem imprecise from the first glance, but can actually be used, I don't think there're any significant reasons against it. Additionally, the recent change was made to make BaseList2D being hashable. This effectively means you can operate with your objects in a dict.
The python tag approach is the least efficient, although would still work, yes.
You're saying your GeDialog plugin uses "mostly the ideantical userdata". If it was "identical", you could potentially use c4d.gui.DescriptionCustomGui with the SetObject function to make it point to your object. This way you can avoid hassling around all the data sync, because it is all handled as a built-in functionality of this class. This is how the attribute manager effectively works, or the Active Object Dialog as well.
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Cheers,
Ilia