XBMC is adding a new offline feature in their Eden release (scheduled for end of 2011). I can't post links yet, but the documentation for this is on the XBMC wiki site; Just search "Media Stub Files"
With this feature, you can create a text file within the movie directory, name it using the movie title and add extra info at the end to tell XBMC that it's a disc and what kind. For example, if you have Finding Nemo on Blu-ray you can create a Finding Nemo directory with a blank text file inside. Rename the file "Finding Nemo.bluray.disc" (making sure to get rid of the .txt extension). You can also add some xml info to the file to give a custom message:
<message>This is on the shelf with the Disney movies</message>
When this gets added to XBMC's library it will show the DVD/Bluray/HDVD/etc. icon in the video information and when you attempt to play the movie it pops up a window saying "Please insert the following disc: <movie title>" and the custom message below that. The window includes a Play button and an Eject/Load button.
To me, this is a better solution than creating dummy video files for everything. I've added the suggestion to the codeplex site.