Actually the .wfs file is an archive.
After you extract it you will find very similar to Android apk folder/file structure.
There is a folder called res where you will find folders:
drawable-nodpi with all your drawable (graphics) files,
font with all fonts you have used in your watch face,
values - this one looks like some remains of the previous GWS features but it looks like it can hold all values for your watch face in strings.xml file
xml with watch_face.xml (only Android wallpaper tag) file
and
watchface.xml where all magic happens - all color options, dimensions, gyro settings, animations etc.
Except res folder there are also:
latest-preview.png file - quite self explanatory
and another “gem” file or more to say “the sweetest” one called honeyface.json where another part of settings are written including font settings and much more.
So in theory those files is ready to be edited
BUT
firstly both watchface.xml and honeyface.json are not easily readable by human being as they are strings of simple tags
AND
make the changes in pure code without possibility of checking the changes in flight is a bit cumbersome.
So for now all we can do is to ask for this possibility/feature to be added to WFS.