The Web Access Control spec is here: https://github.com/solid/web-access-control-spec . In principle, we could support really granular data ACLs based on that spec, the ACL applies to a URL, and you can give any datapoint a URL, so problem solved. In practice, it may become a bit cumbersome, I suppose it remains to be seen how you'd do it in practice, but I think we have a really good start there, there's lots of stuff that can be realized now.
There's a wealth of academic research that applies to these problems, the problem of the Semantic Web community has been that it has been strong on the academic stuff, but not very focused on truly useful stuff that can be done right now (to quote AaronSw). That's one of the things that Inrupt sets out to change. But the good thing is that we're not setting out to solve really hard problems in the dark, since we have that academic research, we know pretty well what's hard and what should be within reach.