sorry, as far as I know it is actually not possible, the manual says the following:
Movies are played in true color (24 bit) mode, therefore use 24 bit movies if
possible. High color movies (16 bit) are also supported, but play slower and look
worse due to the color conversion process.
When you use an alpha channel, 32 bits are required.
I have never seen an AVI file with alpha before, this might be the reason the
feature is not implemented.
maybe you could work around it, by writing your own code....
