It's listed in the update that there's a way to get the alpha or a pixel, but I can't find the command anywhere. Getting the R, G or B is in the expressions of the image manipulator, but alpha's not there.
Huh... weird, that was definitely added to the code, but it didn't seem to get included in the 0.99 release for some reason. Should be in the next build.
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I'd hazard a guess that it wasn't recompiled?
It should be in the 0.99.2 release now, anyway.