I think you should take a look at the LODSync component. It controls the LOD of each components. Under “Components to sync” you will find the Hair and its Sync option will probably be Passive. Change it to Drive, then add an element in Custom LODMapping, name it Hair and add 8 elements (one for each LOD). With this, you should be able to set the LOD of the hair for each LOD of the Metahuman.
I hope this helps and that the explainations aren’t too confusing.