It's not possible to bump the fee from the GUI in a watch-only wallet, yet the option is enabled:
<img width="344" alt="Schermafbeelding 2019-10-26 om 11 26 47" src="https://user-images.githubusercontent.com/10217/67617446-8e62fe80-f7e3-11e9-80a0-fcc70d683c09.png">
It's also thoroughly confused about the fee, but that's not a problem if we don't show the dialog: <img width="348" alt="Schermafbeelding 2019-10-26 om 11 25 14" src="https://user-images.githubusercontent.com/10217/67617436-6ffd0300-f7e3-11e9-9931-05ab3b705c7f.png">