i am of course, less
experienced in this area than most of y'all, but maybe from the outside can offer another view. i can also easily make a fool of myself, but what the heck, nothing ventured, nothing gained.
while not very informed on the subtle definitions of the greneng, which needs to be defined for us new members, and assuming the original poster was basically talking about the ORIGINAL FUNCTION of the notched area at the top of the blade near the ganjah, i note that the rencong also has similar notches cut into the similar area, and in fact on the rear side of the area where your fingers can come in contact on mine (they're sharp little devils too).
the kudi/kujang also has decorative notches on the top of the blade, on my pre-islamic one, they are well forward, over the 3 holes.
10.5 in. blade kujang - 9in. blade rencong
european blades frequently have decorative filework notches cut into the blade spine, scots blades are also frequently notched
4 in. weidmannsheil hunter
could it just be that they are decorative items that have evolved into fancy forms & have become 'traditional' in some areas, adding more esoteric interpretations in later ages.
as not all kris blades have them, and some non-kris blades have them, i'd think that if they had a definite non-decorative kris specific function then all kris would have them to some extent, however they are apparently optional.
occam's razor (razors generally come without greneng