Does the GBC register LCDC ($ff40) bit 0 do anything ?

When the GBC is in GBC mode, this bit does not disable the background or window. When this bit is 0 sprites will always appear above the background & window regardless of any sprite priority settings or tile attribute background priority settings.

When the GBC is in GB mode, this bit operates properly.

If rom location $143 is $80 or $c0 the GBC is in GBC mode.