How to write the default value of an attribute to XML?

Using FM 9.0.3 and a slightly specialized DITA application...

Is there a way to write an attribute to XML if there is only a default value specified for it in the EDD, but no value was entered by the user? I know the reason why FM does not write it to XML by default (e.g. because the DTD may specify a different default value for the same attribute), but anyway I want/need to achieve this for one specific attribute in my structured application.

Are there any R/W rule or EDD based tricks to achieve this, or is an export XSL transformation the way to go?

Thanks for any help,

Marcus