P9P provides a script E which uses B which uses plumber which uses the p9server to communicate to Acme. By setting E as your EDITOR variable to use acme (or sam, if you prefer) as an external editor for applications like mutt, pine, or your favorite IDE. This is very convenient.
Just remember that E will quit right after it detects that the file has been modified, meaning that you can only save your file once. :)