Therefore, there were early attempts to superheat the steam. With low-pressure, low-temperature saturated steam, the steam temperature could be raised quite a bit with success. But as boiler pressures got higher, with the search for greater efficiency, as metals and designs improved, the superheated steam got too hot and the engines failed.
The problem was not that the metals of the boilers, superheaters, and engines could not stand the temperature. The problem was lubrication. As long as engines were lubricated with animal fat (tallow), temperatures were limited to what the tallow would stand. Get it too hot, and it turned to gritty clinker and scored the valve surfaces so they leaked.
Now, you’re talking vegetable oil rather than tallow, and if I read the OP correctly you may be talking about once-through-and-out lubrication. So there is a chance, just a chance, that this could work in a purpose-designed engine. As a retrofit I’m a bit dubious, partly because of the high starting viscosity.
I’d also worry about forming epoxy deposits on the hot cylinder walls below your piston rings. You might be able to burn these off if they form above the rings due to vegetable oil fuel, but I’m not so sure about formation below the rings due to vegetable oil lubricant. That said, castor oil was used as a lubricant in the WW1 gnome radial aero engine, on a once-through cycle, but it has a lower iodine value than say rapeseed or sunflower oil.
Interesting idea though. If you try it, be sure to let us know how you get on!