Hi Gianni,
I thought that dotlock was really close so I added the functionality to
it. The attached patch has an "--append" option that takes standard
input and appends it to a file.
That is very interesting. However, I believe that from the point of view of
mailutils, dotlock is not the right place for it. I'd suggest writing
a separate application for this kind of functionality. This will also have
the advantage of being able to support several locking schemes provided
by mailutils (including dotlock).