Good luck getting out to Freo from the airport and return within 6 hours and not spend an arm and leg on it. You'd be looking at over $120 (return trip) on taxi's alone.

There are quite a few bars which are open in the CBD, but personally I don't know if they would be truly worth the cost of getting from / to the airport for the 4 hours or so you'd have in the city.

Unfortunately 6 hours put's it into the "you could, but it'd be expensive and probably not worth it" category.