You can use partial to render that field. in rails_admin.rb inside model config:
field :<your_field> do
partial "<your_field_partial>" #app/views/rails_admin/main/<your_field_partial>
#You can use sub directory there as expected.
end
The partial: I am posting an example>>>
<%= form.hidden_field field.method_name %>
<% permission_array = @object.permissions.split(',') %>
<% Constant::ROLES.each do |role| %>
<p class="permission_roles">
<%= check_box_tag role, role, permission_array.include?(role) %> <span><%= role.humanize %></span>
</p>
<% end %>