There have only been a couple of ways I have scene leather used for stuff like that but not sure if it would give the effect you are looking for. One method is using epoxy of sorts and leather dust and you make basically a leather based putty that is worked into the general shape then sanded to form the handle. Its more of a rudamentary type style but can be useful. I mainly see people doing it on leather working tools.
I have scene leather used as a wrap style handle. Basically the leather is wrapped and glued at the same time but that would be more for a hidden tang type. Neither of these methods IMHO are as good as the leather disk method for a hidden tang knife.
Theoretically you could either use a couple of thick pieces or several thin pieces laminated together then epoxied on and shaped, not sure how well this would hold up though.
EDIT: totally forgot about all the braided style handles, dont think that will work on a flat type base but I dont have much experience with the braiding of handles and such.