Due to legal requirements, working in any way during a period of furlough (even as a volunteer) is grounds for disciplinary action, up to and including termination of employment.
http://www.politico.com/story/2013/09/government-shutdown-furlough-blackberry-97598.html#ixzz2gxVYDHmm
A decades-old law prohibits furloughed workers from conducting business outside the office. This extends to government-issued mobile phones or dot-gov emails, now ubiquitous in both work and play.
