How to manage merges from one line while excluding its merges from another

I have a tree like this:

      G---H---I---J---K   devel
     /       /       /
A---B---C---D---E---F    main
         \
          L---M---N---O   my


The 'my' branch forked off the 'main' line and for reasons out of my 
control cannot merge the 'main' changes (D,E,F) back.

The 'devel' line is a another line that I need to track and merge from, 
but I only want the changes local to that branch that are not on from 
the mainline. i.e., I only want commits G,H,J.

I want to continue to merge from the devel line as additional commits
are made that are not on the main branch, but I do not want any of the 
future merge commits.

Is there an automated way to do this or do I have to cherry pick 
everything by hand?

Thanks,
Lane