Some of the unschoolers that I've seen just kind of decided to rule out math and science as focus areas, and did so brilliantly in the humanities that they were able to pursue awesome levels of higher education--one who I know personally is currently attending Reed, for instance. C. S. Lewis was no unschooler, but he did not ever learn math, according to his autobiographical writings.
Some did a lot of math early on despite lack of interest, via math games with Mom at her insistence. Is that really unschooling? Not all would agree that it is.
And some were passionately interested in it, and so they went after it on their own, with various curricula and activities.
If someone were really starting from scratch, and not interested previously, I think that I would start a three parallel track program. Track 1 would be math facts, skip counting memorization, and experience of the organic relationships between fractions, decimals, and percents. Track 2 would be logic puzzles and strategy game training to teach that KIND of thinking. And Track 3 would be some specific math curriculum, probably Saxon, starting with level 65, after about two months of the other tracks being pursued daily, using both mom instruction and the DIVE CD's, 6 days per week, year round. I'd continue into 76 immediately rather than taking a break, and encourage the child to test out of the review lessons in 76 to speed things up. Then 87, similarly. That would take, I believe, about 2 1/2 years, and for a former unschooler it would be pure hell because it would be so laden with relentless requirements. I can't imagine a parent who was used to be hands off actually making this happen.
I mean, I have seen unschooling parents agonize over their 8th grader's not knowing cursive, and how it bothers him so much but they don't want to push him but he's decided he can't possibly learn this but it's not right for them to intervene. That is SUCH a different way of thinking than mine. Because basically the reason I didn't unschool is that DD was starting to decide that she was stupid because she didn't know how to read, so even though she didn't want to, I insisted on teaching her. So in the cursive case, I'd be all, look, you can't let this beat you, here, I'll do up a worksheet for you every day and soon you will be a pro. And that would be that. But for someone who has been unschooling to 7th grade, there is a deep resistance to assigning things to kids at all, and it would be a rare 7th grade hater of math who would have the drive and the grit to push through a program like I've outlined on his own.