I've seen a couple people mention using scented Geraniums, but it seems to me they are tall-growing, bushy plants.... could they be sheared enough for topiary? How do you use them?

I have 5' tall topiaries from scented geraniums; you need to pick a tall growing variety like Prince Rupert or ginger.
They do have to be staked but are very easy to keep shaped once you get a nice rounded top. Just pinch to keep in shape and use your pinchings for potpourri or scenting bath water. Logees.com has recommendations for varieties that do topiary well.