Are composite joins possible using FetchXml in Microsoft Dynamics CRM 4.0?

Question

I am using FetchXml to query CRM 4.0. We have a special case that will require a composite join between CRM entites. The FetchXml schema indicates that multiple link-entity elements are allowed, and it also indicates that multiple filter/condition elements can be added to a link-entity. The problem I'm facing is that the value attribute of the condition element does not appear to permit an entity/column name. It expects an explicitly declared value.

For example, FetchXml lets you specify this:

<link-entity name='myentity' from='column1' to='column2'/>

... which does the T-SQL equivalent of this:

JOIN myentity on column1 = column2

And it lets you specify this:

<link-entity name='myentity' from='column1' to='column2'>
   <filter type='and'>
      <condition attribute='column3' operator='eq' value='myvalue' />
   </filter>
</link>

... which is the T-SQL equivalent of this:

JOIN myentity on column1 = column2 AND column3 = 'myvalue' 

FetchXml does not appear, however, to provide an equivalent of this:

JOIN myentity on column1 = column2 AND column3 = column4 

Note the difference. FetchXml provides for conditions in the join, but it does appear to provide for a composite join, that is, a join across multiple columns.

Has anyone out there in cyberspace been able to perform a composite join using FetchXml in CRM 4.0? Thanks!

More information:

I'm hunting an answer that uses FetchXml to accomplish this - not SQL or the QueryExpression syntax. The SQL above is there just to explain the concept.