Good idea! If it's an image, you can watermark "DEMO" or "DRAFT" very visibly over it, and that hopefully will help them realize it's neither real, nor intended to be what is submitted.
I think a large part of the problem comes down to two somewhat related things: people only understand things based on their own experience, and people have very little imagination.
As developers, we spend all day building things that don't have the right or final (or any) content, and we understand that a box filled with lorem ipsum will look fine once the real text (which doesn't exist yet) is in there, or a screen with just a big red rectangle adorned with "PLACEHOLDER 500x300" will look perfect once the final image is inserted. But for consumers of what we build, they have no experience of non-finished products: they don't ever see placeholder images on Facebook, they don't see unwritten copy in the newspaper, and when they open up Microsoft Word, they don't type "The quick brown fox..." into it, they type the actual text they want. We know that the hard work from a development perspective has been done, and it's now someone's job to write the copy and pick the images (which is, of course, no easy job)... but to the end user who has no concept of the code whatsoever, all they see is a page where everything is totally wrong.
Secondly, imagination: if all they can see is a red rectangle, many people will find it very hard to imagine a tastefully chosen hero image there instead. We built a product a while back which, in large part, was intended to be flexible and extremely configurable - that was a really key feature of the software. When I was initially demoing it, I showed it in two configurations: one that was incredibly basic, and one that was a wild mix of various garish colours and fonts. I had anticipated that people would use their imagination to interpolate various tasteful designs between the basic and the garish, but no - all they could see was boring or ugly. Even with a careful explanation - "Of course I've used awful colours, you can easily choose from your corporate palette!" - it just never went down well. (Now when I demo it I show off the configurability by tweaking in real-time two pre-existing designs, very different but both very pretty, and both of which are firmly grounded in real-world use-cases, which avoids nitpicking like, "Well, it looks very nice, but we'd never use it like that, we'd have another foo after the bar...")
To the guy saying to use Bacon Ipsum, just no. That's fine and funny when you're a developer sharing something within a development team, but otherwise for a surprisingly large number of clients it will be confusing and possibly seem extremely unprofessional.
