Eureka! Found the problem!
I downloaded the latest BlitMask, which still threw the same error, which got me thinking "What if they're not negative, but undefined!"
Somehow, the stage was throwing resize events prior to stage.stageWidth and stageHeight being available, or the stage having content. They must have been undefined/null when my resize handler was first fired.
Updated the stage resize handler to check that the BlitMask instance exists and that the stage has content before worrying about resizing, and it seems peachy. (And we seem to get a statistically significant increase in execution speed to boot!) [Edit]
Strangely enough, this error only pops up when exporting for Air - everything worked fine with the same resize handler when exporting for Flash Player 10.
I'm guessing that Air must handle calculating the stage width/height differently, firing resize events before the stage width/height are set initially, or that the Flash Player has some inbuilt logic to avoid these situations that Air is missing.. Too weird!
Thank you for the help sir! You truly are a scholar and a gentleman, and your spirit of generosity and willingness to assist are inspirational, much like your code.
I'm cooking some pretty neat functionality into TransformManager that I'd like to share with you when complete, to return the generosity and good-will you've shown. I would provide it in hopes that it inspires you to completely re-write and greensockematize (greensockify?) my work for future inclusion - is this something you'd be interested in, and if so, where would be best to send the modified source?
Cheers,
Chris