I posted on the other thread, too, Mmelindor. I think the sweaty head (disclaimer: in an otherwise healthy, happy baby, etc etc - DO NOT IGNORE OVERHEATING BABIES) thing is a red herring. most cot mattresses are plastic backed - anyone would get a sweaty head sleeping on one, even in winter (my two did).
trunk/core body temp is what you should go by.
the lightest grobags are the equivalent of a sheet, and have extra big armholes etc to encourage airflow, but obviously they cannot be kicked off (as there is nothing worse than waking freezing cold at 2am!)
I also agree with what you said on the other thread re: how to cool a room off. a lot of people just don't know when to open windows/not open windows to keep the house at its coolest. opening them in the middle of the dya when temps are roasting (inside and out) is not going to help. up early, get the windows open for a bit, to get nice early morning cool air in, then shut in the heat, and open again at night - with shutters ideally, so you can keep the windows open.
I think, given the summers (when they arrive!) are reasonably regularly hot in the uk now, there shoudl be a lot of education done on this.