I've done that. In fact, the last client I got responded to my rate by saying, "We can't afford that." So I said, "I can't lower my rate, but we can see what I can accomplish with 50% of the hours." And I got the gig. We're now towards the end of that contract, and now both of us are in a better position to know what/whether I can deliver what I promise.

Still you have to know what you're going into. If I'm looking for a 40/hr week gig (consulting or staff), I'm going to be less thrilled with 20 hours a week.