What version of AddMusic are you using? I'm pretty sure no music from Wario Land 1 uses the NSPC patch, so it's unlikely any version will cause issues on a real console. I may be wrong about this.
And erm... yes, the Wario Land games did have backgrounds, for the first game, it's the beach water/clouds in the first world and what not, and various other backgrounds later. I distinctly recall Parsely Woods and Syrup Castle having quite nice background graphics personally.
http://vgmaps.com/Atlas/GB-GBC/WarioLand-SuperMarioLand3-SyrupCastle-CourseNo40.png
Note the light grey? I think that's the background.
Wario Land II also had them...
http://vgmaps.com/Atlas/GB-GBC/WarioLandII-Chapter2-InvadeWarioCastle-Story1-ToTheCastle!!.png
Note the trees in the outside area.
As did Wario Land 3. Of course, the backgrounds are usually rather simple.
My advice would be maybe to mix the backgrounds up a bit. Put some of the Rice Beach and maybe Mt Teapot background graphics in one background for example (one ExGFX file) and do that a few times for each world.
For gaming news and Wario discussions, check out
Gaming Reinvented and
Wario Forums respectively.
As for Mario's Nightmare Quest? Well, it's currently on Fusion Gameworks, ROM Hacking.net or the GCN at the moment.