Well, that's actually an acceptable answer. Here's why:
As an SGE, the restrictions on appearing as an expert witness in a matter where the United States is a party are substantially narrowed. You are only prohibited from appearing as an expert witness where you actually participated in the same proceeding or particular matter that is subject of the proceeding. Since the subject matter of the proceeding and your work as an SGE are different, the prohibition doesn't apply to you (unless you fall under one of the categories that are subject to heightened restriction) .
But, can you still serve as an SGE on the committee? If you appear as an expert witness for this organization, it's possible that questions might be raised concerning your impartiality with your current SGE work. (Remember the discussion on "appearance of a lack of impartiality" a few sections ago?)
So, if you decided to accept the invitation after all, you should check with your ethics officials to make them aware of what you are planning to do.
Now that we've examined many of the ethics laws and regulations that apply to you while you work as an SGE, let's examine what happens after you leave Government service...