We are getting ready to ship this functionality in Collect, Central and pyxform! Please take a look at the latest specification if this interests you.
We've changed this to saveto in the XForm spec for consistency with other bind attribute names (e.g. readonly). We're keeping save_to in XLSForm.
We've decided to use an attribute with a boolean value instead.
These initial releases are experimental and we will continue to consider changes if there's a strong reason for them.