Adding the SVN external revision number to the build "Revisions" notes

It’s quite simple to get the revision of an external reference out of subversion, I can put the cmd in a batch file, and redirect the output to a file which I add to an artefact.
I would however prefer that the revision of the external is also reported in the “Revision” marks when I select a specific build.
Can anyone explain me how I could achieve this?

Many thanks for the help.