I doubt that this is possible.. The functionality of tabulator is so "forced" that even if you can get the form to set focus to some other field, tab will still function as always. The guestion is, which happens first: built-in functionality or your custom build..
In some ways, this is also a security issue. Maybe.
BTW. I hate that when I open a PDF form in my browser (firefox) and try to CTRL-TAB to some other browser tab, it doesn't work. Reader "steals" the key stroke in some way..