Seems to work fine just from status menu, without needing keyboard or to reserve proximity sensor for this job. Good!
The problem with proximity sensor was, that even when Camera application was closed, covering proximity sensor brought the lfocus controls in front, for example in Desktop view, Browser view or anywhere.

I do not know if it is possible to know which program is on the foreground, but maybe lfocus could ignore both proximity sensor and keyboard when Camera is not on the foreground, whether or not they are enabled from the status menu.
That way one could use proximity sensor also for other jobs, but for lfocus when Camera is in front. (Would have to ask shortcutd then again ignore proximity sensor when Camera is front.)

The reason I have no lens cover, is because I use the old version of Mugen battery back cover. The old version has space available under the cover. I have microSD-USB-adapter, 2nd microSD card and tiny Bluetooth-adapter all hidden under the cover, for all kind of sudden data transfer needs on the road. Yet to find small enough microUSB-USB adapter to fit inside also to have emergency charge ability wherever is a USB-slot available.

But I can also see that someone would want to keep lens cover and camera application open in the background, to make it quicker available for next use or to continue previously paused video recording.