Harmy said:

Well, I found out that it may not be that hard after all - a lot of the shots done had the round corners but that was before I decided to tighten the cropping by one pixel on each side (this was done for technical reasons - to hide some artifacts), so when this new cropping is applied to those shots using a matte with rectangular corners, the rounded corners are all but gone - only in the upper left corner, there's a tiny trace of the rounded corner left, which shouldn't prove distracting when switching from/to a shot with the rounded corners removed completely.

So you were always going to add a 1-pixel wide black matte over the entire image to hide artifacts? That's fine by me, but were the people who were throwing a fit over the rounded corners aware of this? If so, that's hilarious because it will make those few dozen pixels lost in the corners look minuscule by comparison, as that 1-pixel wide matte will cover up literally thousands of pixels!

Wow! Some people REALLY don't like their corners round, I guess! :)