It appears that people do overclock the K6-III+ 450. One enterprising individual OC'd it to over 600 MHz, though with extensive cooling.
It may be that $15 worth of cooling would make it run fine on 2.2v, at least that's what this article implies:
I don't know if the Scovery case can dissapate the heat, though, or if the flip-up cover would interfere with the cooling.
I'm going to try mine at 1.8 and see if it works. If it doesn't I'll try 2.2 and monitor the temperature.