It's possible to use the recursive property on your template.
Basically what you do is on your homepage, or which ever root node for the section you want to use, you create the property which will contain your sidebar text. I would suggest the homepage.
Then you add a page field on the template with the attribute recursive=true
You can add the page field by adding the property a follows
This would result in a field that looks something like the following
<umbraco:Item field="category" recursive="true" runat="server" />
In my example, the property on the homepage which is being used is category.
Then any page which exists below the homepage, if it doesn't have a property with the same alias, then the value from the homepage will be taken.
A very useful feature :)