There are few methods for applying motion blur to fluids, depending on what effect you are trying to achieve.
For instance, if you're flying through a clouds generated with Maya fluids and the clouds are done by animating a texture offset inside the fluid container, then what you can do is animate the fluid container shaking on sub-frames along the direciton of motion, so that it doesn't look like it's moving on whole frames, but renderers like Mental Ray will still get the motion information and apply it.
That cheats the directional blur. For other scenarios different methods are used.
If you're concerned about motion blurring the whole container and not the fluid inside of it (a moving fluid-based torch flame for instance), then what you can do is make a cube matching the shape of the fluid container and apply 2DMV shader to it, to get a motion vector pass for use with ReelSmart Motion Blur in comp.
You have to understand that the fluid container voxels contain no actual motion information unlike verticies in geometry. Every frame of the fluid container, it's essentially a different fluid. There is nothing to interpret the motion information from other than fluid velocities of voxels. What you can do is take the voxel velocity, pass it through camera inverse matrix and override the color of each voxel based on voxel velocities. Then you'll have a 2D motion vector pass, but even then you'll have to do some additional scripting, to help the RSMB plugin interpret transparency.
The way RSMB works is it looks at the color of the motion vector pass. Green color represents Y direction. Red represents X. The range of values is from 0 to 1. 0 in red channel, for instance, means the pixel is moving to the left at maximum speed. 0.5 in red means the object is not moving. 1 means the object is moving at maximum speed to the right. 0 in green means down. 0.5 in green means stationary. 1 means up.
What if there is transparency? That means the color you see is not really indicative of the direction. You have to divide it by alpha value of the pixel to know what the actual motion vector is... A good example to illustrate the limitations of the 2D motion blur would be a moving window. Make a semi-transparent plane in Maya. Put a sphere behind it. Animate the semi-transparent plane moving by the camera, obstructing the view of the sphere. Set the motion blur to 2D in render globals and hit render. You'll see that the sphere behind our virtual "glass plane" got blurred, even through it shouldn't have, because the motion vector pass doesn't interpret transparency. Change the blur type in render globals to 3D motion blur and only the glass will get blurred, but not the sphere seen through it.
I've written a proprietary motion blur tool for maya fluids, but unfortunately I cannot post it due to contractual obligations to my employer. I can answer any questions you may have to help you write your own tool. Also, I can instruct you on how to program RSMB-like plugin, if you don't have access to the legit thing.
