While that could be done, it's a bad idea.
The handling of the drop (i.e., downloading all the files) is done in the explorer process, which means while explorer fetches all the files, it would be unusable.
For slow connections, that would be much too long.
Also, to initialize such a drag/drop, TSVN would have to list all files and folders recursively first, which also blocks explorer (or any other app you drop the revision onto) completely.
Stefan