Need advice with linksys WRT54GL router

I've seen a WRT54G reach 1km. But this was in a resort village; the router was in the clubhouse, and I was on the balcony of my aunt's house across the small bay. There literally wasn't anything between except the sea air. Yet the signal was very weak and my laptop kept on disconnecting.

And of course, in a more urban environment, that range is practically impossible, unless you're on a roof of another house. Anyways, I don't see any problem with what you plan to do; just make sure moisture will never enter the cable-antenna connection. Also, make sure water won't drip down the cable straight to your router, and that the antenna can stand up to high winds.