A primitive watermark function is no problem. The issue is that as soon as DxO or any other developer add a watermark function, there are another thirty or forty feature request on how it looks, can it be a png, can a logo png be combined with text, positioning, alternate transparency.
DxO is not an efficient tool for resizing for web (where watermarks are necessary). I suggest you find one of the $10 or $20 utilities for resizing tools (higher compression, smaller dimensions, watermarks, removing EXIF) to fill in this part of your workflow. What we all need from PhotoLab are the full size jpegs (for print, for masters for resizing for web) and the TIFFS for re-editing in a pixel editor for advanced masking and combining of multiple pictures (HDR for instance).
Just a suggestion. If you are on a Mac, I can make some recommendations. TillTheEndofEternity made a suggestion for Windows which is free.