Is this possible with tortoiseSVN?:
Always freeze svn:externals
for tags
the scenario is our trunk will always use the \'HEAD\' revision for externa
I would second Michael.
Additionally to recommendation from SVN documentation:
Generally, SVN seems to be designed mostly for code contained in a common directory. svn:externals (even by name) is added for referencing external repositories. So I would expect other problems additionally to mentioned above.
On my experience need for usage of svn:externals for non-fixed revisions frequently comes from wrong structure in repository.
May be it's not your case, I just advise to take into account these points...