It would still require template modification to integrate the custom fields and template modification is the best solution. Since that is not feasible for you, the least we could do here is use jQuery to move content from one area of your site to below your entry-header like this.
To do that, add the content in a Text widget and then add this code in Theme Options > JS
text-5 with the ID of your widget (this is how to get the ID of your text widget) and replace
.single-post .entry-header if you wish to implement it in single posts only.
Hope that helps.