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You need to make your attribute scriptable for it to round trip throught IDML.
AFAIK in order to save your custom textattribute in inx/idml you need to provide scripting access for it, because that's the way the export/import engine works – in fact, it makes use of scripting and inx/idml files aren't much more than scriping instructions to re-create InDesign document. So, you have to make it scriptable...
Thanks Pawan and Maciej for your replies.
Indeed, i was just examining the Framelabel example. Combined with your answers, i see 'the light' now! ;-)