I’m sure the rest of you figured this out ages ago, but it was a new realization to me. In addition to supporting multiple, different borders per image, the LR2/Mogrify border options for Lightroom 2 allow for borders to be applied on three metrics: pixel size, percent of height, and percent of width. That allow, for example, for easy application of letterbox-style top and bottom borders without needing to pay any attention to the cropped dimension of your image.
The plugin default is in pixels; just switch to percent and you’re off. Something in the range of 10-12% of height seems to suit my eye pretty well.