Use a form to populate a deeply-nested hstore

[sscirrus]

I'm trying to figure out how to use fields in a form to populate parts of my hstore field.
Here's the data in the hstore:

custom = {
  "settings" => {
    "search" => ?,
    "start" => ?
  }
}

What works (not ideal):

<%= f.text_area :custom %>

What doesn't work (and what I need)

<%= f.text_field "custom[settings][search]" %>
<%= f.text_field "custom[settings][start]" %>

I asked this question last night on SO but no answers yet:
http://stackoverflow.com/questions/34470564/use-a-form-to-populate-a-deeply-nested-hstore