Many of the watch faces let you customize which complications are accessible. And which apps are available seems to vary some based on the watch face. I just don't see any that allow you to do the camera remote. Maybe next update.
I haven't found a third party camera app with a companion complication for the Apple watch that controls the iPhone camera, but the GoPro app has a complication to control a GoPro.
I understand these are two completely different things, but just thought I would mention.
Not quite what I was looking for but good to know! Thanks. I've seen a couple of apps that allow the AW to control their app. But nothing on if they allow it to be added as a complication. And I'm not keen on spending money to find out right now.