Avraham asks his servant Eliezer to place his hand under his thigh to swear an oath regarding finding a daughter for his son. Chazal explain that he asked him to hold his (Avraham's) milah (male member) while swearing because it was a mitzvah item (similar to how in a rabbinic court one would need to hold a sefer torah) and that mitzvah was beloved to Avraham.
But was this really appropriate from a tzniut perspective? Moreover couldn't Eliezer have held the stone which Avraham used to perform the bris milah instead? Furthermore there is no indication in the text that the mitzvah of milah was more beloved to Avraham than, say, the mitzvah of welcoming guest. Why not instead have Eliezer swear while holding onto the entrance to the tent?
In short, please explain the necessity of the unusual process of this oath beyond the basic explanation given by the commentators.