Unfortunately my experiments are to agree with your conclusion Graeme. I have struggled with this for some time but not found a way of computing a String via Fn::Join and keeping it for later in the template.
Can I share with you a typical example?
As at 2014-09-18, the most sophisticated way of storing a string value for re-use in the Resources section would be via a (second-level) Mapping. Better still, I can pre-compute a Condition and determine whether to use the string or not. But unfortunately the storing of a string just does not work.
Here is an example stanza of such a Mapping …
And here is an access expression to it in a Resource
In this example, Environment is a literal string Parameter with a convenient default. So that is a very clear example of creating and re-using a string. But here is what the CloudFormation processor says …
Template validation error: Template format error: Every Mappings attribute must be a String or a List.
So I share your frustration that there does not seem to be a way in a template to provide the effect of a temporary string variable.
… So far as I can see. But the answer you originally received said the opposite of this (although without an example).
So if anyone does have an example workaround, then please could they share?