Is there a word or phrase that means the opposite of disciplined, but which doesn't have negative connotations?
Perhaps free-spirited is what you want:
free spirit n. One who is not restrained, as by convention or obligation; a nonconformist.
However, I'd say that "the opposite of 'disciplined' ", if one can be agreed upon, is bound to have negative overtones because 'disciplined' has positive ones.
As hinted at in Edwin Ashworth's good answer, consider also unrestrained
Not restrained or restricted: a display of unrestrained delight
It can be used positively, negatively or neutrally.
Another term that can have a range of inflections is wild. While it often has a pejorative tone, it can be used in an admiring way
unconventional; fantastic; crazy: wild friends
It also can be used neutrally
Lacking regular order or arrangement; disarranged: wild locks of long hair.
A person who doesn't behave in a ritual manner or doesn't follow a fixed routine could be described as being easygoing. It usually carries very positive connotations, and implies the person is patient and tolerant, whereas disciplined can suggest intolerance and inflexibility.
Easygoing: Not rigorous, demanding, or stressful: "an easygoing life as a part-time consultant."