You can actually simulate the autocast-toggling behavior of the real pet buttons with a macro that can be placed on a non-pet button:
Code:
#showtooltip
/petautocasttoggle [btn:2] [mod:shift] Growl
/cast [btn:1, nomod:shift] Growl
This will cast Growl if you left-click the button or use the button's keybind without the Shift modifier. If you right-click the button or hold Shift while using its keybind, it will toggle the autocast status of Growl instead.
Depending on your actionbar addon, you may or may not get the autocast and autocastable overlays on the button to tell you when Growl is autocasting. You should, however, get at least the icon and cooldown.