Controlling high current (1000A) with MOSFETs
Look at page 4, fig.12, graph of safe operating area. That is exactly what you need.  



You are talking about single pulse, right? You didn't mention any repetition or timing at all.
If you open mosfet hard, say Rdson is 0.85mOhms. In case of 1000A the Vds will be less than 1V, so you have to look at the left side of graph.
There is no line for 100ms pulse, so you have to interpolate between DC and 10ms pulse. The safe current is much lower than 1000A. It is like 400A. And it is the maximum.