I made a very simple patch to add MULTILAYER image type to both command line rendering and python API.
With this patch, it's possible to make a batch render (using -b command line option) and outputing to an OpenEXR mutlilayer file (same as selecting Multilayer image type in rendering panel). To do that, use "-b MULTILAYER" option.
This patch also contains the addition of MULTILAYER as a python value acceptable for setImageType function on RenderData.
Finally this patch also correct a little mistake in source/creator/creator.c where R_IRIS was used instead of R_TIFF when using "-F TIFF" on command line (making it impossible to render to TIFF on command line).