I wrote an advanced filter based by extending the structure that you've proposed. The current one is compliant with yours and it would be great to have it in a 3.8.1 release .
It looks very interesting. We consider to add such feature in the near future.
To be a honest with you – IMHO it look a little complicated for the “End User” – maybe it should look a little simple and the most important – it should be integrated with jQuery UI Theme Roller.
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