Thanks for the information.
sslstrip can already change or remove headers. Look into the python code and you will see that you can strip the headers before they get to the victim.
I could not find any examples of sites actually using this. I think that google and other famous sites are now hardwired to use https in the browser and don't need to use the Strict-Transport-Security
However if you are pentesting for a client that is using Strict-Transport-Security in their headers, it would be a simple matter to remove it by modifying some python code. If you an find an example of such a site using it. Then I would be willing to try to modify sslstrip.