It is certainly possible, but it might be easier as a contractor, with a company helping you line up projects. I got started in QA working for a company that did this very thing, lining up contracts with clients for test work, then finding Automation or Manual resources to complete the contracts. Some of the projects were short (just a couple of weeks) to some that exceeded 18 months.
I know you're asking about freelance, but it certainly shows that companies are willing to hire automation resources on a project by project basis.
Update: I just did a search on "SDET" and "freelance". Several sites came up, donanza, elance, and freelanced (not linking as I know nothing about any of them). They seem to be sites for freelance developers to find projects, but also contain postings for SDET projects.