As far as I know, you can only do UA or AA to Starwood, with Diners as an intermediary. However, you get crushed value-wise going from DC -> SPG, so I probably wouldn't do it unless you are just topping off for a very big Starwood hotel award. (Especially if you would have to pony up a Diner's annual fee to open an account with them.)

As for what most of us have thought about: running a ton of miles through SPG at an acceptable loss and then back out to your favorite airline at 1 -> 1.25, it isn't possible unless you are willing to accept 1 -> 3/8ths, which is what you'd get using Diner's (even with the SPG 25% bonus on a 20K transfer).