Indeed, i opened the tiff in bitmap editor (masks were additional layers) and used another image as background. I wasn’t precise enough with mask in darktable, so on few places i tweaked the mask a bit in bitmap editor.
( I used different bitmap editor tho but that’s not so important, you can use krita/gimp for the bg replacement or to convert the multi-page tiff to different format as e.g. psd for use in bitmap editors which don’t support such tiff file. I did also patialy copy hair from left side to the right as there were lacking volume there. )
The reverse approach is possible too. You can e.g. misuse the watermark module to bring different image to darktable.
I used it here for sky replacement here: Colour Grading Exercise - #30 by danny