If you are open to capacitors (and I am not doing the calculations- 300kW for 30 secs seems rather large) a possible technique used in industry for holdup in power supplies is to use higher voltage (along with higher voltage capacitors of course) since the energy stored is C*V*V/2. Of course in a switched power supply there is already a high voltage at the input, so in your case you may have to boost it to 200V or more and then "buck" it back down when the power is required. I doubt that this is practical for your application, but it will reduce the number of capacitors and perhaps allow the use of electrolytics.