Thumbnails: Add black bars

I’m generating thumbnails using base code. Most of the images I’m uploading are 16:9 and are just resized. Some of the images, though, are not 16:9, and are being cropped.

Is it possible, instead, that if an image isn’t exactly 16:9, to resize the image so it’s width fits the thumbnail size, and add black bars to the top and bottom of the image to make it 16:9?

(Obviously with the above, 16:9 is just the aspect ratio I’m using, I’m sure someone wants the same solution but “100px wide” instead =) )

Base code? Some customization for generating thumbnails? Plugin?

I’d even be fine if there was a checkbox during upload the said “this image isn’t 16:9, you best be doing the black bars with this one”.

Thanks!