gday james 
 
i have never heard or seen microwaved bonsai mix 

 but i cant see it not working, the microwaves would smash just about anything in not to long so i cant see why it wouldnt work, i would wait for it to cool down for an hour or so before using it though.
if you need to buy a microwave check out the second hand shops or even the recycle centres at the local dump!
i wouldnt even feel sorry for the curl grubs, it would be interesting to see what happens though at a guess they would most likely explode.
(i know that sounds cruel, but insects/bugs dont actually feel pain or anything like that, they work on these things that tell them what to do (cant remember the name of the body part but its the goo you see when they splat on the window in the car, they only know what they need, eat, work, breed) so i wouldnt be to concerned about it all. even when you see the squirm when they lose a leg or something like that it is just the elictical current that goes through them. 
its like when you see a group of ants going for scraps and one or two get stepped on and are still "alive" the rest of the other ants go at the injured one as not to waste..
sorry, i know that got a little off topic..
jamie 
