According to some posts in the thread of Magical Dimensions Zone, I saw that you know how to use FOFs... so, create a transparent and non-solid FOF will be very simple.
All you have to do is create a simple FOF, as if you want to create a FOF with the linedef type 100 (Solid, Opaque, Shadowcasting) except that the linedef must not be 100, but 221 (Instangible, Translucent).
Now, the solid wall FOF has been replaced by a translucent and non-solid FOF.
The texture of the linedef 221 of the control sector is the color of the translucent light. In your case (because you want a white light), you should use the texture XMAS20.