You can't use Tab key for shortcuts, at least in Xfce Terminal.
Direct answers
[...] it's just shortcuts that involve Tab that are a problem. I assume these are special to GTK and don't even reach the application.
Your assumption is correct. The limitation actually applies to GTK+ application programs and Xfce Terminal is one of them. And it's not just shortcuts that involve Tab key, but also few other keys i.e. PageUp, PageDown, F10 that cannot be used individually (without modifier key).
Is there a way to override this?
Likely no, although workaround may exist. A better way is to look for alternatives to Tab key, or use other application programs that allows it. See also the following explanations.
Explained by contributors
First, there is dated bug 3524 that reported "Next/previous tab keyboard shortcuts does nothing". The most relevant comment has been quoted below.
[Comment 9: Nick Schermer 2009-10-10 13:49:08 CEST]
Xfwm and terminal use a different way if handling accelerators (xfwm directly from x, terminal relies on gtk). is in gtk handled as an invalid accelerator, so to 'fix' this terminal will check if the accelerator is valid before setting it. As a result pressing will do nothing in the shortcuts editor from now on.
Second, there is dated post on Stack Overflow that asked "PyGTK: Allow usage of Tab in AccelGroup". Relevant part of question has been quoted below.
When using gtk.AccelGroup any combination with Tab charater is invalid. Now I do understand that UI navigation is done using this key [...]
Explained by documentation
Regarding "UI navigation is done using this key", GNOME Developer documentation has noted the matter in topic of Keyboard input as quoted below.
Keyboard navigation
[...] Tab is the standard key for moving around an interface with GTK+ and GNOME.
Additionally, shortcuts that use Tab key have dedicated functions.
Standard navigation keys
Tab and Shift+Tab
Moves keyboard focus to the next/previous control
Ctrl+Tab and Shift+Ctrl+Tab
Moves keyboard focus out of the enclosing widget to the next/previous control, in those situations where Tab alone has another function
Ctrl+Tab and Shift+Ctrl+Tab
Moves keyboard focus to the next/previous group of controls
So earlier comments make sense now.
Revised answer (2017-04-04)
Despite Tab key is recognized as ^I
in the command-line shell for tab completion, it has least relevance with inability to use Tab key as shortcut in graphical application programs.
Why Tab key cannot be used as a shortcut? The reason was simple: Tab key and its combination keys are reserved by GTK+ and GNOME.
Despite GNOME Terminal via GSettings allows to use Tab key for shortcuts, there is no equivalent hack for Xfce Terminal via Xfconf to this answered date.