Looks like Virtual Texturing is handled through the normal texture system, and is controlled by a cvar (defaults to being on). So I guess you can load the huge textures directly in the normal way.
static TAutoConsoleVariable<int32> CVarVirtualTextureEnabled(
TEXT("r.VirtualTexture"),
1,
TEXT("If set to 1, textures will use virtual memory so they can be partially resident."),
ECVF_RenderThreadSafe);