While researching another issue (trying to prove whether or not tgb should be castable on pets) I discovered something else.
It turns out in 2003 patches there is a note:
Target Group Buff - TGB will no longer work with any illusions.
This seems to mean prior to 2003 TGB should be working on group illusion spells, i.e. share wolf form.
I've yet to put together further evidence (as I said I'm currently researching a different issue), but this alone seemed worth flagging for attention.