Sand blasting would be safe, if done properly. There are many different abrasives that are use, some would chew up Al, others will gently strip it away. A good shop will be able to select the proper type of "sand" use.
The cost will vary. You may get the "nice guy" discount - as in we won't change for such a small piece, we'll just toss it in with some larger car parts.

Al will come out a dull grey colour, you can polish it up with some 0000 steel wool. To keep it bright and shiny, I would clear coat. You use a spray paint option - cheap and easy. Even if you don't polish it, I would recommend throwing a coat of clear coat on it either way. It should only take an hour or two to spray on the clearcoat and for it to dry. Not really a big deal.

Now that you mention it... I'm thinking that I might try powder coating water bottle cages for my next bike that I hope to build for the spring/summer season. A touch of customization.

~Monkey~