Hi Maurice, thanks for looking into it for me. Shame there's not more info. I guess I was assuming that it was a characterization of some native deity instead of a generalized symbol.... I did some online reading and it was mentioned that the Aso was "generally thought to confer protection upon those who possessed this image".
Unfortunately that leaves the face motif unexplained, but I can see how the "demon face" symbolism works as an icon to for the enemy to fear.
The Pit-Rivers Museum has some further info on these shields (still have more to read myself). I did a search for "Dayak Shields" and got a few pages of info - a nice online reference for those interested: