Technically I could put a .105 steel rod on my bass, just set it there from nut to bridge really, and it would be a very low (zero if is just sitting there) tension string, but I wonder how comfortable it would be to fret...
I happen to have a 0.125 stainless steel rod that is 36 inches long. That is real close to the diameter of my C string. I did not try to lay it on a bass but I did prop it up over a board with the "action" set quite a bit higher than I normally would. Pressing it against the board at the first fret location did take more force than you would want to have. Everywhere else it was very easy to fret actually, much easier than any bass string I have played. Seems to me that even with a "string" that stiff the perceived stiffness is the result of string tension. The real issue with a string that stiff is that if you fet it at a low fret it automatically frets itself half a dozen frets up before you get it against the fret you are trying to hit. As a result you could not play that string even though the fretting force is not excessive.
Ken