@GilbertoA - Hi Gilberto - I ran into the same issue with the length of the string when I started looking into this question. I have a workaround that isn't really pretty (no es bonita!), but I think it will get the job done.
This workaround involves using a hidden field to store the value of the Base64 encoded string, then uses the action set on the hidden field to post the value to the clipboard. I've attached a document with step-by-step configuration instructions. I hope it helps!