for instance, we have a field "sidebar" for nodes. of course, that field's content should be rendered outside of the normal "content", in a different region. - Alex 18 hours ago
Instead of using the theme's layout and side bar region, you could use the one of the following modules and create a panel or display suite layout with a side bar region. Then you can insert the node fields and other blocks you need on the sidebar and on other regions that you create.
###Panels
Panels
The Panels module allows a site administrator to create customized layouts for multiple uses. At its core it is a drag and drop content manager that lets you visually design a layout and place content within that layout. Integration with other systems allows you to create nodes that use this, landing pages that use this, and even override system pages such as taxonomy and the node page so that you can customize the layout of your site with very fine grained permissions.
Display Suite
Display Suite allows you to take full control over how your content is displayed using a drag and drop interface. Arrange your nodes, views, comments, user data etc. the way you want without having to work your way through dozens of template files.