I recently created a custom child entity that I wanted to show both on the left side navigation and on the parent entity form itself in an Iframe.  Here are the steps I took to accomplish that.  This will also work for non-custom entity views that you would like to embed into a form.  This will display a full view in the contained Iframe with all the same button navigation as if you went the view directly in a new window.

Step 1 :
Add an IFrame to your form and set the source to about:blank.

Step 2:
Open a parent entity in a detail view.  Select Ctl-N to open the detail view in a new browser, now view the source and find the view you want to embed in the left hand navigation.  Once you locate that in the HTML source grab the contentarea references in the loadArea("") function.

Step 3:
Add the following script to the onload event for the form that contains the Iframe(parent entity).  Replace the 'contentarea' with the value from Step 2.