I wanted to add a control that would add some html to site collection administration pages. These pages are not directly editable.
I looked at adding controls to the additional page head tag within the master page. The control in the master page looks like
<asp:ContentPlaceHolder id="PlaceHolderAdditionalPageHead" runat="server"/>
To the project add a new module
<Control Id="AdditionalPageHead" ControlSrc="~/_CONTROLTEMPLATES/yourcontrol" Sequence="15100" xmlns="http://schemas.microsoft.com/sharepoint/" />
The sequence number is just a random number.
Now your control will be added to all pages that use the masterpage. Just place some conditional logic in your control looking for the site collection addition page and there you have it