I use DeoVR and it has a pin option in the settings. I personally haven't tried the SLR app because I let kids use my headset commonly. I'm not sure if it supports using a pin or not but if it doesn't you maybe happy to hear about the next update for the Quest 2.
I just checked the current meta release notes here and it doesn't look like they have added it yet but supposedly it will be an added feature in a update sometime this month for the Quest 2.
I'm not sure if this is exactly what your asking about or if your asking for a way to completely hide the icon for the app itself which would be a great feature maybe it could just look like something else when active it could be a calculator app with a calculator icon and then you would need to enter a certain pin and hit the = button 3 times to open the SLR app from the calculator app. I don't know maybe the new Meta pin update will have something to hide apps but I don't know about that just have heard about pins coming.
