If you're using flowgraph to begin the sequence, add a Debug:ExecuteString node, and make it execute the CVAR "r_motionblur 0" when the sequence begins, and another node to execute "r_motionblur 1" (though if you set it to 1, the motion blur is rendered as it is on "low" settings. A setting of 2 or 3 is medium and high, respectively) when the sequence ends.
That's the only way I could think of doing it,

Here's what it should roughly look like: