We autopost to any blog running metaweblog api, which square space does. I was able to add a square space site to my test account and autopost to it fine, but others have had issues. email us if you need to do this and need help
I was expecting them to have an RSS feed input for their blogs on their end, but it doesn't seem to work that way.
You guys should license your services to them as I'd be happy to pay a few dollars each month to have your kind of easy input mechanisms over there automagically. (Yeah, I know it's free now, but it doesn't feel like it should be.)