Hi Tim,
I'm Paul McLean and I am the Google engineer who is mostly responsible for text in LayOut. What you are trying to do is a great idea and entirely reasonable. The hitch is that LayOut stores text in the document in RTF format. RTF does not support alpha (transparency) values for font color, so that information is lost when the internal text engine generates the RTF for LayOut to store. Granted the UI shouldn't be giving the user the expectation that alpha is supported for text (this is a logged bug and not as easy to fix as you might imagine).
A work-around might be to create a "water-mark" image with transparency (PNG supports this) and overlay that on the page. Put it on a layer by itself and it will stay out of your way when editing the document.
Regards,
Paul