Hi Jason,
If I understood correctly, you want to store the live url of a page in a form field (defined in the doctype of the page). You can do this by using the 
treenode event handler, or if you use workflows, the 
workflow event handler.
You can create a method that adds the live url for the page in the designated field in the doctype. Then call that method in the OnAfterInsert (treenode) or OnBeforePublish (workflow).
Getting the page url can be done with
TreeNode currentDoc = (TreeNode)treeNodeObj;
string docLiveUrl = functions.GetUrl(currentDoc.NodeAliasPath)
I don't know for sure if the Functions namespace works in the custom event handlers, but you can try.
Hope this may help.
Best regards,
Bram