A few thoughts of analysis-
It would be unusual for a cabin opening to be doing nothing.
Is it in the front or back seat area?
A friend had two identical forward facing air inlets fore and aft on the fuselage side. Air came in the forward one and went out the aft one (going frontward).
It sounds like the only "known" is that it doesnt have a forward facing scoop, and that it is assumed to be an inlet. It might be surprising to confirm what it is actually doing in flight with a string, in climb and cruise and descent, probably all the same.
Assuming there is another "inlet", tape this one closed and see how the other inlet works (with no exit).
A cabin inlet needs an outlet. Since it doesnt have a scoop, it may be functioning as a needed exit, in either area. A flush slot can create low pressure, creating an outlet.
I have two inlets, one in the front seat on the leading edge of the strake, and the inlet for the back seat is under the strake/fuselage corner with a 5/8 inch forward facing opening, faired back along the corner about three inches.
They are both robust inlets-- because of an excellent exit on the opposite fuselage side under the aft armrest. A slot exit as you describe would work there, but i made it an aft facing NACA to give folks something to point at and say "That wont work." :)Oil flow and string flow shows it to work interestingly well. The location under the armrest results in no exit noise for the back seater.
During Oshkosh lunch Nick Ugilini described several interesting and useful results from pressure testing his LEZ cabin using an airspeed indicator. His bottom line was that it is significantly pressurized, showing 68 mph of pressure at one point. If flyingwaldo can further describe the slot, I hope you can chime in Nick.