I use physlock to lock my screen. In order to use it automatically from xfce4-power-manager, I had to create a script named xflock4 (since it's the first command that xfce4-power-manager checks), and launch physlock from this script. So it works great, but it was a little bit tedious to set up, since I had to grab xfce4-power-manager source code, look into it to understand how locking is done, then create the xflock4 script. It's a little bit hacky, and not everybody can do that. I imagine that letting the user specify its own lock command in the GUI could be helpful to some users. Does it sound like a good idea ?
We considered it, in the end it's not really something you want to expose to the average user. Personally I think the average user shouldn't have to worry about *which* locker he uses. Either way, this is implemented in xfce4-session, you can set a hidden property called "/general/LockCommand" to the command you want executed and xflock4 will be disregarded instead. I would say we won't implement this feature in multiple places, that would just make it more confusing (or we would always have to sync these settings), so for now this will only work for users of xfce4-session.