The engine seems to only allow picking which runtime to use on launch. First it checks the -hmd=name command line parameter and uses that if found. Otherwise it will pick the one with the highest priority based on the HMDPluginPriority list in Baseengine.ini.
[HMDPluginPriority]
WindowsMixedRealityHMD=40
OpenXRHMD=30
OculusHMD=20
SteamVR=10
The runtime names are defined in the different classes implementing the IHeadMountedDisplayModule interface. Most of them have aliases set up so slightly different names may work as well.
To use them on the command line, add a parameter like: -hmd=OpenXRHMD
Here is a list of all runtimes I could find:
SimpleHMDMagicLeapAppleARKitGoogleARCoreHMDGoogleVRHMDOculusHMDOpenXRHMDSteamVRWindowsMixedRealityHMDDefault