My two cents:

- complementing the post by Bob Kuykendal, and as an example, the air
passage though the tailfin spar of an LS8 is comprised of three small
holes with a combined area of barely 3 square inches. Added to the other
constrictions along the way, this means the ventilation pressure drop
occurs mostly after the cockpit. Thus, of course the cockpit will stay
significantly above ambient pressure in an unmodified LS fuselage.

- regarding the reingestion of ballast water (or pee...) at the end of the
tailboom, perhaps it is linked to lower pressures at the top end of the
rudder hinge? The location of the horizontal tailplane on the Genesis
would suggest suction occurs there.

- finally, and after applauding the designers of all these fine new
outlets, perhaps the next step is to locate the inlet in a neutral or even
a low pressure area? Why, you may ask? Because there is no reason in
principle to pursue the highest possible ventilation pressure drop.

With a nose inlet and a turtleneck exit, the total ventilation pressure
drop approaches twice the dynamic pressure of the outside free flow (i.e.
the pressure coefficients may approach +1 at the nose and -1 at the
turtleneck). The power lost to the ventilation flow is the product of this
pressure drop by the flow rate, e.g. at 100 kts a flow rate of 20
litres/second costs 30 Watts. This power is subtracted from the
performance of the glider.

If the inlet is located instead in a neutral pressure area (and the
cross-sections are suitably sized), the same cooling flow will cost only
15 Watts - and the cockpit will achieve an even lower pressure than
before, which is doubly good for performance!

Going further: an inlet may even be located in a moderately negative
pressure area (I envision exchanging the pop-out window for a small
naca-entry connected to a small eyeball vent). The Cp at this location is
about -0.7; with partial pressure recovery, perhaps we get -0.3 in the
cockpit. As the pressure at the turtleneck exit remains even lower, it is
still possible to create an effective airflow. Result: the most
energy-efficient ventilation possible.

Sounds counterintuitive, but should work and be easy to implement in a new
design (existing designs may be constrained by the impossibility of
increasing the cross-section of inlets).