Decals printed with ceramic powder laser printer is the only and easiest method I have ever known. I contacted with largest enamel color producer in USA and France to able to print art on aluminum guitars and they gave me the temperatures is needed. Few years ago , these enamels were leaded to create stronger colors but they are banned in europe and usa. Now all enamels are lead free and more pastel .
I think diluting with a solvent and print on aluminum plate with silkscreening and fire at the open fire or torch is only homemade option to create oversized images on AL without need of kiln is only option. But they warned me to keep flame safe to avoid warp.

Umut