I would try a ControlAdapter, they are quite powerful for changing built-in things. So set it up to alter EPiServer's edit string property control, find the textbox object and change input to hidden (maintain id, name, and so on so EPiServer will persist it into the database). Then add your own buttons and scripts for opening dialogs, entering text and saving it in the hidden input field. Haven't tried it on EPi's string property, but I did a job on a FileUpload control once; there is an example and codesample here.
Edit: Oh, i read EPi6, perhaps some other approach is better for 7..
I solved it after I found a google map selector guide that I used as a direction for what I needed to do.
When Im in the epi editor I should instead of a textbox for a string property display a button with the text "Click to edit". When I click this button a event should append opening a popupwindow rendering an .aspx page with a textbox in it. When I hit save on that page the value I typed in the textbox should be added to the "hidden textbox" in the epi editor and close the popupwindow.
Does anyone have any tutorial or example of doing this?