Yes, but there’s a third class, the disposable virtual machine template. Which is really an appvm with a special flag set. So if what you’re looking for exists on AppVMs, it probably exists on disposable VM Templates too.
You just reminded me of a rant I want to write about how things are named and described in menus. I still get confused sometimes as to what will happen if I try to open (say) an xterm on something listed in the menu as SomeVM-dvm. I will either get a named disposable named SomeVM-dvm, a numbered disposable (disp1234…), or actually open the disposable virtual machine template itself. I’m going to argue that we need a new naming convention in menus.
Those qubes should be based on a dvm template. If you change that template, you change the disposables spawned from it.
It might not be obvious what that template is. You generally get a disp1234 because you did something in some other qube; if you open the settings for that qube (or, command line; qvm-prefs), you’ll see an entry (on the gui it’s on the advanced tab) default disposable template. THAT is the name of the template you will want to do this on. The trick will be getting the template to open an XTerm (or whatever tool you want to use to do the modifications), rather than to have it just create disp666 for you and open the tool in that (useless for present purposes). Since I don’t know what your menuing system looks like I can’t necessarily help. (If you’re on KDE, it will be labeled "Template (disp): "). I simply don’t recall what the xfce menu calls that, and I think you might have to enable it to even show up with a special flag in qvm-prefs or qvm-features.
On the command line, though, you can type "qvm-start " then go to the blue qube (qui-domains tool) on the menu bar, select it and open a terminal. You’ll actually be in the template; changes made there will persist in disp9876 qubes and you should be able to do your xen wizardry there.