Unusual position, that last illustration of yours Alan. Ergonomically we dont vertically load the wrist much in that position. Interestingly there has been some work on using a hiking pole in almost exactly this position with a grip not unlike the keris grip. The research suggested that it was a good position to take load in, so it is very likely you could use a keris quite efficiently with this grip.
http://www.pacerpole.com/index.html
drd