Transparency in Windows version?

I'm trying to see if PsychToolbox (2.54) is up to an experiment I'm
planning. I need to be able to take odd-shaped fragments of images
and assemble them onscreen. Having some means of doing transparent
colors is going to be a must so that the rectangular windows holding
the fragments don't end up overwriting bits of the other fragments.
But, it seems as if the copyModes in the Windows version are currently
very limited, while the Mac version has a nice set.

Is there another way to do the transparency? I suppose I could do it
manually in Matlab, but that would seem to defeat the purpose and
would likely be slow, no?

Craig