Sorry, no, you can't, unless the whole AROS and all of its apps, current and future, accessed memory in big endian mode, much like Amithlon does. That would of course mean a big speed penalty. Such a penalty is ok for an emulator, it's not ok for a general purpose OS and its applications.
I'm not quite so sure whether this would really have such a big speed impact, Fabio. It's hard to say, but I would think 95 % native speed for most applications might (if someone tried) not necessarily turn out to be completely unrealistic.