> Base64-encode it?

Then you'd have to convert UTF-8 encoding to real unicode codepoints in the script, and that's a pain in the ass too. The real trick is that you have to use the %x1234 URL encodings for unicode characters in the cookie strings, but that might mean you have to write your own cookie handling code, AND you have to convert character sets appropriately, especially if the board is not using UTF-8. In PHP it probably depends on which version you're running.

> What if we don't use Windoze?

Linux has some sort of alt-keypad trick too, I forget the details. It's much the same as in Windows, though. Or just cut-and-paste!

> No, I want the field to be appear as absolutely empty, not with a space in it.

There's a few zero-width unicode characters you might use, I think... I remember coda playing with that on some test installation of Kareha. It was some sort of RIGHT-->LEFT switching character.