Call macro with new root

I want to be able to call a macro with a given node used as the effective "root"
for the called macro.  For example, given the data model:

(root)
  |
  +- user = "Big Joe"
  |
  +- latestProduct
      |
      +- url = "products/greenmouse.html"
      |
      +- name = "green mouse"

I want to have these macros:

<#macro Product>
 ${name} (${url})
</#macro>

<#macro Describe>
 ${user}'s latest product is <@Product=latestProduct/>.
</#macro>

Such that the result is:

Big Joe's latest product is green mouse (products/greenmouse.html).

I have looked into ways to achieve something like this using
TemplateDirectiveModel to writing a directive that re-roots the environment, but
that does not seem possible.


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