The whole point of growing outside the way he describes is to make it look natural -- Inconspicuous mushrooms sprouting in the back yard.As opposed to an OBVIOUS indoor operation.

Now having said that let me add that a 3'X6' plot wouldn't be that inconspicuous. A HUGE rectangular plot with hundreds of mushrooms fruiting all at once is going to catch poeple's eye.I think you should use smaller plots and shape them irregularly so they don't look so cultivated when they fruit.

And, as far as I can tell from what I've read, plain worm castings without straw seem to be your best bet as a substrate.

Have a look at this if you haven't already:

http://www.sporeworks.com/bulktek.html

Also innoculating with grain outdoors might be tricky. You might be better off getting castings started indoors first then use the castings as spawn for your outdoor beds.