I wanted to get a diff output between a particular revision of a versioned file and an external file.

I understand this is possible svn diff --old "path/to/versioned/file1"@1234 --new "path/to/versioned/file1"@1235 find the differences between revision 1234 and 1235 of the file.

And also this svn diff --old "path/to/versioned/file1" --new "externalpath/to/external/folder/file1" which finds the difference between the latest versioned file and an external file which is not versioned with SVN.

What I wanted was to compare a particular revision against and external file which is not versioned.
Something like this:

svn diff --old "path/to/versioned/file1"@1234 --new "externalpath/to/external/folder/file1"