If your case is windowed, I would try to keep the color scheme of the painting (brushwork?) similar to the hardware, that could look very cool probably. I am a sucker for dragons or phoenixes myself. I always wanted to see a high quality version of this:
http://i203.photobucket.com/albums/aa242/cc_68333/phoenix-1.jpg and it might be quite good on a case with a black/yellow or black/red scheme.
EDIT: Ah, you are the one going for those cobber wrapped tubes etc. In that case, I would probably go with something in the general steampunk colour scheme. Brown, cobber, grey, silver, brownish orange and red.
Perhaps something like this:
http://www.clubchopper.com/forums/attachments/paint-tech/35777d1307064620-toss-up-couple-more-pieces-my-airbrushing-steampunk1.jpg
A steampunk dragon is also possible:
http://fc06.deviantart.net/fs46/f/2009/200/f/5/Steampunk_Dragon_by_kerembeyit.jpg
http://fc04.deviantart.net/fs70/i/2010/189/4/e/Clockwork_Steampunk_Dragon_by_zerojinx.jpg
Google image search is your friend for all this