gdal - How to set opacity with gdal_rasterize

I am using gdal_rasterize to create a raster of a huge file containing polygons. I managed to generate the raster file with the right color and with the minimum size to tile it after with the gdal2tile.

gdal_rasterize -of GTiff -co COMPRESS=DEFLATE 

-b 1 -b 2 -b 3 -burn 136-burn 46 -burn 17
-co PREDICTOR=2 
-co ZLEVEL=9
-ot Byte 
-a_nodata 255 
-a Niveau 
-ts 500000 500000 
-l layer W:/layer.shp
W:/layer.tif

Tile will be used to overlay Google Maps so the polygon must be transparent.

Has anybody experimented with this usage of gdal_rasterize and succeeded in setting the opacity of the output raster?

Answer

Sorry for the short answer but I suppose that simply adding an alpha channel might do the trick. At least I could make an output that is semi-transparent in QGIS with the following command.

gdal_rasterize -of gtiff
-ot byte  
-co alpha=yes 
-burn 255 -burn 0 -burn 0 -burn 100 
-ts 500 500 
-l test test.shp test.tif