I’ve thought about that, and don’t think it’s possible for essentially 2 reasons
1) Legacy status doesn’t facially touch a protected class. Race is a protected class, so the consideration of that is how they killed AA. So I’m not sure there’s an actual constitutional violation to allege under the 14th amendment in the same way there is when race is used.
2) make the round about argument, due to past discrimination and longstanding effects of racism. Legacy admission is basically a proxy for race, and that consideration of legacy admissions implicit also considers the now banned use of race. Which runs into problems because there are minority legacy admits. And even I could sell that theory. I don’t know how to get around disparate impact analysis. Which is basically if a policy seems to solely hurt black people, so long as no one is on record saying the n-word and expressly saying we did this to hurt black people, no harm no foul.