can I use svn:externals instead of svn copy to create a tag?

[Gary Hallmark]

All examples I have seen for tagging use svn copy (or svncopy -tag) to 
create a tag. If I want to make sure nobody can change the tag, I guess 
I could write a commit hook. But couldn't I use a versioned external 
definition to create the tag instead of svn copy? Seems this would 
better enforce that nobody can change the tag, since it isn't a copy at 
all. One downside would be that if the tag contains unversioned 
externals, I would not be able to pin them down, but I don't plan to 
have unversioned externals. Are there any other downsides?

Example:
trunk/
tags/
   X -r123 ^trunk version_1.0
Cheers,
 Gary