You're welcome. :) I'd put in a white colour fill layer at the base of the layer stack and then export the TIFF file as a JPEG. You could then just switch it off again or delete it once you're done with the exporting. Not ideal, but it would work.
True, but OP is talking about TIFF originals and transparent areas of an image, so I don't think it's too much of a leap of faith to make the assumption that PS (or some other external editing application) might be involved... I can't think of how you could have a transparent area in an image edited solely in LR?
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