This has been reported as 1912134 – Feature Request: Compile with face-detection enabled. Took a first look, and enabling the face detection component for the Fedora package doesn’t seem to be too complicated.
One question however: While the face detection component itself isn’t that big, it draws in a good amount of dependencies. Therefore, I’d like to move it to a sub-package that can be optionally installed. Now, how will main Shotwell behave when that subpackage isn’t installed and thus the dbus service isn’t present? Will it handle that situation gracefully?