The very tip of the anvil horn is your friend in the world of scroll riveting. Almost all rivets can be backed here, and once you get the hang of balancing your piece while riveting, you'll be fine. If you really need to maintain the dome head, then obviously this wont work. But on a large piece no one will ever notice the difference, especially if all your rivets have the same flat spot.
In some cases, you either can't balance the piece at the right angle, or the space allowed isn't big enough for the horn. In these situations I like to use a large bending dog or scroll wrench to back the rivet. This works best with a friend applying pressure, but can be done solo if you're clever. You can get very creative with this and will find that you almost always can find a way to back it. I have included some pics of a single malt scotch rack I made last week that was entirely constructed out of rivets. Note the position of the bending dog to back the rivet head.
Also, when it comes to actually hammering the rivet, don't be afraid to come in at weird angles with the ball peen if you can't get a straight shot. Just remember to alternate directions so you don't bend the rivet too far over one way. This technique also requires a little practice before you try it on a final piece, but is invaluable once you get the hang of it.
I personally love the look of scrolls with rivets on them, but collars a great too because some joints just don't look right aesthetically with a rivet.
Hope this helps!