I decided to test k-oxalate in the MW oven for myself.
First I measured dielectric properties of 0.1, 0.5, 1, 2, 5 and 10% solutions

Increasing the concnetration of k-ox increases the "loss factor". Thus, heating is more efficient at higher concentrations.
A 1% solution will heat 3 times faster than water, and a 10% solution about 5 times faster.

In an old crappy box I have here (it's almost shot), I was able to heat 1 cup (250 ml) of 1% solution from room temp (about 25C) to 80C in about 1:00. In a household MW box, it should take about 20 sec

So beware of the speed of heating, and start with short times. Use a half empty container to heat, it'll avoid spills.