Just posted in another post on this topic: I can confirm Metahuman plugin is available for 5.1 in the Marketplace now! Not sure when it was released but looks like sometime in early-mid January.
Btw it was possible previously to import Metahumans using Quixel bridge in 5.1, however it wasn’t possible to do Mesh-to-Metahuman and other features that required the Metahuman plugin, until the plugin was released for 5.1. Which it is now! Thank you, Unreal and Metahuman team!