Hi Roland,
The split-horizon command only applies to distance vector routing protocols (RIP and EIGRP), not to multicast. You can configure the interface to forward multicast traffic that you received on that interface with the NBMA mode:
However, I don’t think that works for AutoRP traffic. If you have the RP and MA on one spoke, then probably the only way to make it work is either by tunneling or using sub-interfaces.
Rene