assign from a function in a RUL

How do you assign a property in a RUL (not a DTL or BPL) from a function value?  I've tried with the package/classname, without, with .., without and even looked all the way down to the parser but still can't see how.  There is no examples and the documentation doesn't help

So this works

<assign property="RuleActionUserData" value="&quot;A01&quot;"></assign>
 

This doesn't complile

<assign property="RuleActionUserData" value="##class(SomePackage.SomeClass).EventTypeToHL7Type(Document)"></assign>

SomePackage.SomeClass extends EnsRules and the method is a classmethod and final