Hey. I was looking for the same answer and reaching this thread too. But now I came up another working method. So I would like to share it with everyone.
So, If the case is your model is not lay down on the floor. You have two model that have almost the same shape but different topology, Totally different model. But you need to bake a displacement map of the high poly model shape into the low poly nice topology with UV one.
So first, Since you can still bake displacement map from multires, Add Multires modifier to your low poly model
Then, Add Shrinkwrap modifier after the Multires. Set your high poly model as the target. And set mode to Target Normal Project so the vertex are not slide but projecting along the normal direction.
Next, Hide your high poly model and hit subdivide in Multires as many time as possible until you satisfy with the detail. And then apply the Shrinkwarp modifier.
Finally, Reduce Preview level of Multires to 0 and do the multires displacement bake.
(Go to bake section in cycles rendering property, Check the Bake from Multires check box, Set bake type as displacement, Make sure you create the output texture in the node editor and select it correctly. Then hit Bake)
Now you can remove the Multires modifier if you don’t think you’d use it later.
I notice that this is very fast to bake. But the result might not look perfectly match the high poly model if you can’t subdivide it enough of time. But if you gonna use the displacement in Cycles that require the subdivision modifier, You couldn’t get any more detail in the render result than the baked texture anyway. It’s most likely to be use with the experimental adaptive subdiv that you still have the same limit of subdivision numbers as you do in Multires. So It’s better to use displacement map as displacement only. And use normal from baked normal map that could get you more detailed shading result.