I can't think of a good way to add the \ at the end when directories are selected, except by writing some VBScript to do the job (which seems like overkill but if you really need it it could be done).
By the way, Clipboard COPYNAMES={filepath} is wrong and only works by chance. See HOW TO: Copy selected filenames to the clipboard for the correct usage.
There's also Clipboard SET which can set the clipboard to any string. Clipboard SET {filepath} is valid and does the same thing as the basic Clipboard COPYNAMES, except that the SET version won't properly handle more than one selected filename while the COPYNAMES version will.