+AD4 So the solution is to just use +ACI-xscreensaver+ACI by jwz. Which can be
+AD4 configured to just blank the screen etc. as wanted by the op. See
+AD4 also
+AD4 the FAQ: https://www.jwz.org/xscreensaver/faq.html
+AD4 
+AD4 HTH,
+AD4 -dnh
+AD4 

Except I use xscreensaver myself and it in no way prevents VT switch,
which is what the OP was hoping to find a way to do if and only if the
screen is locked.  Programs that grab input still don't get to block
combos that are processed by the X server before they even get to the
program's input queue any more than grabbing input will block the alt-
sysrq kernel-level interrupt keys.

Disabling VT switch by the X server and then setting up some other way
to trigger a switch that will be blocked by whatever screen locking
program the OP wishes to use is the best solution I can think of.  chvt
would be the program to call.  Whether he wants it to be a keyboard
shortcut handled by his DE or some other trigger method is a matter of
taste.

LMP