Even if you change the datashape to add your property the service should return it. One possible solution is you can write your own service (you may call the current service within yours) like wrapper which can return your custom datashape along with custom property. You can copy the current datashape and modify it. Then copy the container mashup (or mashup itself if needed) which has this Grid. Make sure to use your custom service to populate the tree. It should work.
I hope this helps.
Greetings Aldo Barletta
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