Thanks for your response.

I think it's slightly more general than you suggest, because the "tax" is so high.  For example, if you're trying to decide whether to buy a larger house or invest the difference in a 529 plan, it could be a better idea to buy a larger house.

I went to one of these schools, and my parents noted at the time that under the right circumstances, they might have saved money by buying a fancy car "instead of" saving for my college.

This post describes part of the cost of college as a "100% [marginal] tax on your post-tax income".  Have you seen other financial-planning and -optimization resources mention this and deduce things from it (whether in an EA context, as you have done, or not)?

There are lots of financial planning resources online (including at places you frequent like lesswrong), and many of them discuss saving for college.  But it seems like people generally appear to have missed a minus sign on the utility of saving for college (in certain situations, as you describe).  For all the words people say about, say, how HSAs are the best investment vehicle available, they don't seem to say as much about how financial aid works at high-end schools.  I'd be especially interested in anyone who's competent enough to run a simulation with this factor in mind.