Vayechi, a unique form of swearing
My own thoughts on the parsha were this: Yaakov exacts a form of shvua from Yosef that we only see one other place in TaNaCh, and that is when Avraham exacts a shvua from Eliezer. They both request that oath be signified by the other person placing his hand tachat yereichi. Rashi explained in connection to Avraham that it was the milah that he wished to have him swear by. It was their equivalent of the practice of swearing on a Bible as a physical manifestation of a mitzvah. I was thinking tht there could be something else going on in connection with the milah and both exacted promises. Though Avraham's primary concern was getting Eliezer to find a bride for Yitzchak, he also explicitly stated that his son not be taken out of Israel. Likewise, Yaakov's request for burial was to be returned to Israel after his death.
The bris milah is associated with taking our place in the land of Israel, as mentioned in the connections said in Birchas Hamazon. Accordingly, I thought it's possible that swearing on the milah is about exerting oneself to make the efforts required that will keep that connection to the land -- even on behalf of another.