if you go into one of the old VW service books from the 60's, there is a bulletin in there for 1200cc engines, if they experience "pinking" (pinging) under load, VW said to drill holes through fins. I don't have book at hand, it's buried at home, so i don't know size, how many, etc. And no I have never tried it.
I looked this up in the old VW service book....for 40HP motors with old round boss 1200 heads....VW suggested drilling 10mm holes (6) in the ribs that extended from above rocker cover rail (upper) to outward most fin. It was done to lower head temperature and reduce pinging on heavily loaded motors.
In the seventies it was very popular to do this modification on moto-cross bikes, they were all aircooled at that time.
the ratio is
should be less than 2s, =thickness of fin, D=diameter of drilled hole.
Ex:if you drill a hole that is four times the thickness of the fin you have the same cooling area, but if you drill a hole that is two times the thickness of the fin you have 50% more cooling(exposed) area.
The thickness of a cooling fin on an original type one head is aprox 2.5mm, with the ratio D=4s, shows us going bigger than 10mm hole decreases the cooling area, I think that 5mm hole could be a good starting point.
Other thing to consider is the effect of the reduced conductive area trough the aluminum for the heath to quickly migrate from the hot areas around the chamber further trough the fins. It will be maybe better to increase the number of holes towards the end of the fins. In theory the concept seems valid. Now we need some real word testing.
Just food for thought?