What if you specify it as an IBA in the XML?
For example, I have the following set in one of my OIRs that sets the value for my XPOSASSY attribute to "Default":
<AttrValue id="IBA|XPOSASSY" algorithm="wt.rule.algorithm.StringConstant" ignore="false" force="true" final="false">
<Arg>Default</Arg>
</AttrValue>