Don't got the field repec droid but I do have the field mail droid and that one can't be used in warzones. I'm guessing it's the same with the field respec droid. But nothing stops you from respec before entering the warzone. I don't see people doing it because without dual spec it will probably get old fast to fill your skill tree every warzone.
It's not a droid. It's a button at the bottom left corner of your spec screen. So you can do it in warzones.