In Roma it seems not to be possible to change @xml:id e.g. to make it obligatory for a certain element.
Right now, if I try to do so (the normal way) and select "change attribute", optional=no, e.g. contents=data.key and submit, I don't see the success message but "Attribute names have to be alphanumeric and have to contain at least one character." and with red background the "Add a new attribute" field.