There should be a way to specify the X class of the terminal. Simply calling gtk_init earlier would achieve this, or you could add another option.
Stupid question maybe, but why do you need to set the wmclass? Terminal doesn't use Xt/Motif, so it cannot be used for Xresources, and Terminal windows all have a unique role, so its useless for the window manager, session manager and related tools as well.
I have a borderless terminal at the top of the screen which I can show/hide using the keyboard or when another window gets mouse focus. Using FVWM, the easiest way to uniquely identify this terminal is by its class. This may not be directly related to XFCE but is quite a small change; shouldn't GTK apps support GTK options?
You'll need to specify the options to the first instance of Terminal when using D-BUS (atleast according to the GTK docs). But the wmclass/wmname properties are really outdated. Today window managers should identify the windows by their roles.
Resolving to INVALID. Window managers should use the role.