I think that you should look at this post and Wiki entry. I think that they will get you started on the detection and insertion portion of what you are needing.
http://devcentral.f5.com/Tutorials/TechTips/tabid/63/articleType/ArticleView/articleId/18/How-to-parse-a-URI.aspx
http://devcentral.f5.com/wiki/default.aspx/iRules/URI__path.html
I'm not sure if I understand what you meant by This iRule will need to scan POST data from /thispage.php and POST data from /overhere/thispage.php. I know that you can detect the incoming source page and URI, the "POST data from" part is the part I don't think that you can do. You can manipulate what you have (add, remove, modify), but I don't think you can collect data from a different page in a POST in an iRule, so I am assuming that you want to detect from "/thispage.php" and just modify the URI to post to "/overthere/thispage.php".
http://devcentral.f5.com/Forums/tabid/1082223/asg/50/showtab/groupforums/afv/topic/aff/5/aft/9545/Default.aspx
Hope this helps get you started.