Good question... I'm not sure about that.
You mentioned building a custom Debian Wheezy Template for Qubes to use for the ProxyVM.
My comparable experiences so far are with the official Qubes Debian Jessie Community Template, which currently does not have ProxyVM support built into it.
Although, Qubes developers have mentioned to me that it would be "easy" to add that to their Debian Jessie template.
Reference discussion: https://groups.google.com/d/topic/qubes-devel/fuB64BGq1so
So based on the fact that their Debian Jessie template doesn't seem to automatically work as a ProxyVM, it might not automatically do what is needed in your custom Debian Wheezy Template without you developing that.
However, the Qubes devs are helpful on the mailing lists and could probably point out exactly what you would need with this, if it doesn't automatically work.