Hi Clemens,
You can't access that property as it's marked as 'internal'. But there are a few things you can do to get at the contents of the replacement dictionary although they are not trivial.
I don't know about what you're trying to accomplish but I did code a "dynamic template" feature for Clarius SFT to support conditional unfolding of project templates. In order to achieve I had to have my own IWizard extension (you can check at the docs for this in MSDN to get to its details). This is what GAX does (its the UnfoldTemplate class you have mentioned) in order to support expansion of template variables from GAX recipe arguments. In my case, as I wanted to still have support for this, I ended up extending GAX's UnfoldTemplate class instead of creating my own from scratch.
This way you will be able to get a chance to peek at what's coming in the replacement dictionary.
Let me know if this helps,
thanks,
-Victor.