dynamically adding derived EOAttribute

In the past, in order to execute a count(*) query, I've added an attribute 
to my EOModels such that it's a non-class property, derived attribute, 
defined as count(*). Now you can create an EOQualifier, create an 
EOFetchSpecification, set it up to fetch raw rows, including only the 
counting attribute, and you've easily executed a count(*) query.

So far so good, but I thought the next logical step would be writing code 
to automatically add this 'count' attribute on demand at runtime, so I 
could execute count(*) queries on any entity I want, without having to add 
this attribute to the EOModel for each entity.

I didn't think this should be too hard; I just create the EOAttribute at 
run-time when needed, and add it to the relevant EOEntity. I looked in the 
.plist file for one of the entities where I had added such an attribute 
in-model, and copied exactly the key/values in there to figure out how I 
wanted to define my new EOAttribute in code. The problem is, when I try to 
execute this code, I get a weird exception from EOF:

sqlStringForAttribute: attempt to generate SQL for attribute 
'_derivedCount' on entity 'ConcreteStoredRequest' with undefined column 
name. You must define a column name for this attribute before attempting a 
database operation.

What? But it's a derived attribute. It shouldn't need a column name. My 
in-model attribute works fine without a column name in it's .plist 
definition.  Why isn't it working when I create it in code at 
run-time?  Below is the code I use to create the EOAttribute. Further 
investigation reveals that the setDefinition() method does not appear to be 
"sticking". No matter what I call with setDefinition(), the result of 
definition() is null. Huh?  Why can't I set the definition? Is this a bug? 
Is anyone else doing anything like this?

             EOAttribute countAtt = new EOAttribute();
             countAtt.setName("_derivedCount");
             countAtt.setDefinition("count(*)");
             countAtt.setExternalType("int");
             countAtt.setValueClassName("NSNumber");
             countAtt.setValueType("i");
             entity.addAttribute( countAtt );