I don’t quite believe these stories but the obvious conclusion is to start your own consulting company where you are not limited by employment contracts…
As an employee, there are emails, trainings, staff meetings, backlogged items, and a million other things that can take up your time. A manager comes to understand your output and sets his expectations accordingly. If you already have another full time job, you do what is asked of you but never more, they expect that from your line of work.
Additionally, as a consulting company you rarely have 'Active Clients' for years and years paying you to do full time work. It's a cycle with outside consultants and you always have to be 'Selling' outside of whatever retainer you charge.
Having two jobs provides a sense of 'stability' that consulting doesn't. But underneath is always the unstability of the 'if the employer found out'.
My friend was working two full time jobs and a couple of hickups caused him to miss a staff meeting for company A. So, he decided to fess up about his other employment at company B. Since he did it kind of akwardly ("Asking for a one-on-one with his manager and saying he had something to tell him"), the manager expected the worst. Come to find out the manager was relieved when he told him, becaause he though he was going to quit! Fresh on the success of Company A accepting his dual employement, he decided to tell company B. Company B fired him and threatened to sue him.