As stated here, int hostfxr_get_runtime_delegate(const hostfxr_handle host_context_handle, hostfxr_delegate_type type, void ** delegate); can be called with the value hdt_load_in_memory_assembly for type parameter.
But it is really unclear what kind of delegate will be returned. By the name we can assume that is some kind of function that loads an assembly from memory, but the method signature can't be found anywhere.