1) Friction- there's nothing you can do to lower the friction to equal bearings. Nothing.
2) Grease- the tubing must be slightly oversized to allow for grease- however, due to the kart's weight, the shaft will always be smashed up tight to the top of the tube. This will quickly hog out the top portion of the tube, and/or wear down the axleshaft. The sloppier the fit gets, the faster the wear occurs, until there's enough play to allow a bend or break
3) Bushings- you could use bronze (oilite) or nylon bushings, but by the time you've replaced them a few times you've paid for bearings, and then there's the labour to rip it all apart quite frequently for regreasing, which won't slow down the wear by much anyhow. AND you'd need to engineer a way to retain these bushings in the tube.
4)None of this addresses lateral loads either- you'd need thrust washers or thrust bearings to provide a bearing surface as the axle tries to move side to side in the tube. Proper bearings don't require this.
5)Debris- When it gets in the tube, and it will unless you find a way to seal it ($), that will chew away at the fitment.
Seriously. You're building a go-kart. You've had to source everything else- get some bearings.
Surely all the other Aussies around here have found something reasonable.