xml to use this is something along the lines:

Code:
	<Fixture menuname="martin RGBAW" uniqueid="MAR6" excelexportid="MAR6" pixelwidth="6" pixelheight="1" output="CUSTOM" comment="Martin RGBAW" channelcount="30" channelsperpixel="5" orientation="TOPLEFT">
		<channel index="1" output="RED" />
		<channel index="2" output="GREEN" />
		<channel index="3" output="BLUE" />
		<channel index="4" fixedoutput="0" />
		<channel index="5" fixedoutput="0" />
	</Fixture>
this goes into the fixtures.xml file.

this sets the aw channels to zero.