I did some more experimenting, and it looks like motion blur on the fluid surface will work - and without any modifications to the FLIP Fluids addon!
Although we cannot modify the velocity attribute directly, we can transfer and scale the existing flip_velocity attribute and transfer this to the velocity attribute using a node setup:
Enabling motion blur in Cycles as well as on the fluid_surface object properties, it appears this does render with correct motion blur vectors.
This will not work for whitewater particles at the moment. I am not sure if there is a feature in geometry nodes to transfer attributes to instances yet, but there could be. I’ll look into it. If anyone knows, let me know as it has been some time since I have kept up with the geometry nodes development.
Note: the velocity attribute feature is hidden in the latest FLIP Fluids 1.1.0 due to a bug (T88811) preventing this from being considered as a stable feature. This feature can be unhidden from the FLIP Fluid Surface panel by enabling the Developer Tools option in the addon preferences menu.
Here is a basic .blend file setup:
geometry_node_motion_blur_setup.blend (1.3 MB)