I guess it depends on how you want to use the poly. If you want to export to a game engine it might be more efficient to use the image sequence directly in the game as a vertex offset(displacement is more efficiently stored as a 2D offset array, or image). If you want to render in Maya then simply creating a matching displacement on a plane should work fine( the heightfield allows you to quickly preview the displacement animation in real time). If you need to poly surface in Maya to model against, then you could convert displace to poly for that frame. For rendering to another package you may need to convert displace to poly for each frame, or use blendshapes.
Out of curiosity, would you get what you want if we had a convert heightfield to poly with construction history?
Or would you rather just have the ability to assign a shader to a heightfield and render it directly(in Maya).
Duncan
