Inverse focused source

Hi everyone,
I frequently use the focused source (/gps/ang/type focused) with a focus point and an appropriate surface from which particles originate.
Is there a smart way to have a designated area being irradiated from said focuspoint?
up to now, I manually calculate the needed geometry, and place it “behind” the focuspoint… I would fancy a way of “reversing” the beam direction, and start from the focuspoint.

what I have:

/gps/particle gamma
/gps/pos/type Plane
/gps/pos/shape Rectangle

/gps/pos/halfx 6.53 mm
/gps/pos/halfy 7.53 mm
/gps/pos/centre -4100 0 0 mm

/gps/ang/type focused
/gps/ang/focuspoint -4000 0 0 mm
/gps/pos/rot1 0 1 0
/gps/pos/rot2 0 0 1

what I fancy:

/gps/particle gamma
/gps/ang/type focusedInverse

/gps/ang/focuspoint -4000 0 0 mm
/gps/target <physical Volume> 
/gps/target <Plane (with appropriate halfx/y and pos)>