[Puppet Users] Reassigning extlookup_precedence for extlookup?

I am still using extlookup with Puppet 2.7.19 - I will not be upgrading to
3.0 and Hiera until closer to Christmas (sorry).

What I was looking to do was use extlookup to retrieve some values using a
particular setting of $extlookup_precedence and then using those values to
set $extlookup_precedence later. As variables can not be changed once
assigned I wrote a class to do this however it does not seem to work.

For example, a simple example in my site.pp fails (being unable to find any
mention of role via extlookup):

$extlookup_datadir = "/etc/puppet/manifests/extdata"

class test

$extlookup_precedence = [ "nodes/%{hostname}" ]

$role = extlookup("role")
}

include test

notify {"Role: $test::role": }


However if the code in class test is merely transplanted into the global
scope it works fine (e,g, role can now be found even though only the scope
has changed).

Is it that extlookup only looks at the global scope for
$extlookup_precedence and therefore there is no way to effectively reassign
this variable based on previously retrieved values?

Thanks in advance ...