There's currently no way to do it, except in person.
Well, there is a way... Each time you reserve a seat, it's blocked for about 10 minutes. If you're really determined, you can be assigned a seat, ask to change, get assigned a new seat, and keep trying until you get the seat desired. It's far from ideal, but does occasionally work (unless, of course, it's trying to avoid assigning adjacent seats on a quiet train, and you're trying to be sat next to a colleague...)
The downside is that you can effectively "sell out" cheap fares by doing this too much!