The problem with this is that it's kind of dependent on what theme the user is using. If you make a font transparent for dark theme users, it's going to still stick out for regular users and vise-versa. Also, if it's transparent so that no users can see it... how is anybody going to see it and know to report it? The whole point of the watermark is so that it is visible.
On that matter, there is a hexcode for transparent text which should solve the problem... Or well, there should be, but we can't use it in Scribbly's text editor.
Because like, the Hexcode in question is
[COLOR=TRANSPARENT][/COLOR] (or at least that's how you'd write it in BBCode), which always makes the text invisible regardless of background.
... Scribbly's text editor only accepts hexcodes like this though:
[COLOR=#FF0000][/COLOR], which stops you from being able to use the codes that involve writing out a word... T.T
So... It should be possible, but with the way Tony-chan set it up to always auto-include the #, it ended up being unusable... orz