Hi Will. My few cents on straw burning...
As you read up on effective pyrolysis, you will see that there is a strong relation between the size of the combustible material and the flow rate of air pulled through the downdraft gasifier. Also the particle density is significant as mentioned by Chris, to produce a persistent reactive char bed to produce a quality syngas. To me it seems that straw particles will not be the right size or density to work in a conventional downdraft gasifier, unless it was very small scale.
There are viable commercial systems used for serious greenhouse heating using an updraft fluidized bed. Another commercial system I am aware of used a refractory lined swirl burn chamber, straw being metered into the chamber, another similar concept used to burn rice husks.
I think the turbulent combustion methods will be best for complete combustion, the fluidized bed approach could produce syngas, but it's not likely to be as good in quality as from a downdraft wood system. Although I wouldn't rule that possibility out with good design and control, as straw certainly lends itself to steady state operation.
I am attracted to the idea of chopped or hammer milled straw. As you say, it's very cheap, and accessible, an over abundant waste material with tremendous heating potential, and with great potential for continuous automated systems. I don't think it's exaggeration to say that straw could transform the energy situation in North America.
Happy experimenting, and looking forward to more discussion,