On aluminium, I'd be inclined to use a clear lacquer first, preferably one designed for aluminium rather than a polyurethane. Then apply a subbing coating of plain 10% gelatine with a drop of hardener added. The Black Magic & Liquid Light gelatins are basically the same type of coating used on photographic papers and are intended for printing on to assorted substrates - If you use one of these, you'd need to "fix out" the silver before using it for cyanotypes....
Unless you wanted to do a cyanoptype over a B&W print.
As for alternatives to regular printing, the idea of printing a resist mask on to a sheet of aluminium and then anodising could be <ahem> entertaining with multiple colours.