I did this in IE, but it's probably similar in other browsers:
- Navigate to the list whose editform.aspx you want to change
- Right click on any item's link to it's display form and choose 'open in new tab'
- Go to the newly opened tab and click the 'Edit Item' button to open the item in the editform.aspx
- append &ToolPageView=2 to the url
- Add a Content Editor Web Part and point it to your document from step 1.
This shouldn't be a replacement for all customized EditForm.aspx's by any means, but it's still a useful trick for quick fixes that don't justify custom development.