Sorry, I forgot to reply to this.
I don't see why not, as long as there is a good flow of rainwater. Ie, if it doesn't rain for 2 weeks in the summer, the water in the butt may start going a bit green and yucky- I wouldn't give them that!
We did use to have a barrel- an open top one that just sat underneath the end of the downpipe, and we used to give water from that to the males. No ill effects- and the goats always preferred the water better from our field which was partially rain water.
Beth