Making a macro library part of a data model?

I've got a bit of an odd use-case.

We've got a custom data model that changes behavior based on user 
parameters:

[@aFancyDirective version="123"/]

Depending on the version number, we want to import a particular macro 
library into a template so it's available to the designer.

We had been doing it using Configuration.addAutoInclude(), but now that 
we need to implement different versions, it would be a lot cleaner for 
us to handle it in the data model.

Right now, our data model looks like this:

public class DataModel implements TemplateHashModel {

   public Object get(Object key) {

     if (key.equals("foo")) {
       return someStuff;

     } else if (key.equals("bar")) {
       return otherStuff;
     }

   }
}

We'd like to make it so that if key.equals("someMacro") then we can 
return the macro. Is it possible to load the macro library into a 
template, and then somehow find the macro in the template and return it 
in response to a get()?


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