I would love this feature too. It would make The Dude's Map almost as good as Visio
There is a workaround
I have been using:
1) Use Microsoft Paint (or similar) to draw things like racks on a blank canvas (I use 1450x1050 for my 24" screen - allows for side and bottom Windows Start Bar).
2) Save the file as .png (or .jpg or whatever you like)
3) Copy the file into The Dude's "data\files\images" folder.
3) Go to Dude, open a map
4) Choose Settings
5) Set Background
to the image you created.
6) Make sure that Tile
7) Go to General
and set Default Zoom
Click on Apply
9) Wait for the Dude to update the screen (usually happens immediately, but sometimes can take a little while).
10) Place your Devices
on the rack (you may have to resize them a bit)
The Dude is very good with scaling, so if another display is bigger or smaller (8" like NetBook or 54" wall screen), everything still looks right.
An additional improvement to adding the Block (either transparent or with z-order) would be the "No Shape" option for devices, so that there will be no coloured background. The only problem with this is that the Shape is used to display Green=OK, Red=Dead. So Dude would maybe need to Red-out the device's image when it fails; or maybe have a green/red flag on it somewhere.