Why would the DC version be any better for supporting the grid. Surely the only difference between the AC and DC Powerwall is an inverter and since all batteries are DC and the grid is entirely AC in Australia an inverter has to be employed there somewhere. The essential requirement to support the grid in "times of need" is the ability for the electricity supplier to be able to switch the battery on or off. It won't be Tesla emptying your battery, but their software would allow the power retailer to draw it down, hopefully at a higher rate than the current FIT.
IMHO this should be in conjunction with an owner determined price/level of charge matrix so you can say "if my battery is above 90% charge I am willing to sell my power back to the grid for less than the standard retail price say 20c kWh" (Obviously if the battery is 100% charged and the panels are producing more power than I am using I have to accept the current FIT for the excess). If it is between 30% and 90% I am willing to sell for a bit more than the standard price say 35c kWh, if the charge level is between 10% and 30% the electricity retailer wants my power they will need to pay me 45c kWh but the last 10% of my battery charge is not for sale because as it is my insurance against blackouts.
There may be weeks or months where the electricity retailer doesn't want any of your power at the prices you are asking, but you will still be able to use the power yourself at night effectively valuing it at the retail price and even if you don't use the full DOD of the battery (which sacrifices some potential income) your battery will last longer. The price/charge matrix could be user adjusted on a seasonal, weekly or daily basis depending on supply/demand levels.
