Redmine

Hi All,
I need some additional functionality.
I want to restrict access to documents inside projects( especially customer should not have access to internal development documents).
My idea for this is to add a custom field "internal" to document categories and to create new category "development internal", in which this boolean value is set to TRUE.

now I found relavant code for displaying documents in \redmine\app\views\documents, which is displaying the categories:

<h2><%=l(:label_document_plural)%></h2>

<% if @grouped.empty? %><p class="nodata"><%= l(:label_no_data) %></p><% end %>

<% @grouped.keys.sort.each do |group| %>
    <h3><%= group %></h3>
    <%= render :partial => 'documents/document', :collection => @grouped[group] %>
<% end %>

but I have no idea, how to access the new custom field in document's category!
please can anybody help? later on I will improve the functionality and I want to commit it as plugin. ( I found some tickets, which are asking already for such a feature)

thanks
Richard