GIMP :: Generating Transparent Watermark PNG

I want to generate a transparent embossed text watermark .png file that can be batch-applied in Lightroom. I get how to apply a bump map to an image inside GIMP, but I haven't figured a good way to get an otherwise transparent .png with just the watermark.

Ideally, this would be totally transparent except for lit areas (white, at varying degrees of opacity) and shaded areas (black, with varying degrees of opacity). In GIMP, I can apply a bump map to anything opaque -- lightening and darkening the color, I guess -- but I can't apply it to a transparent layer.

If there's no straightforward way to do that, I imagine I could bump map onto gray, turn the resulting light grays into a mask for a white layer and the dark grays into a mask for a black layer. The content of the watermark will change, so it would be outstanding if there's a simpler way to do this.