Hello David,
I don't think you can use a POS interface as source regardless of using EoMPLS or not.
SPAN is always used between LAN interfaces: POS frames cannot be sent out a GE port because they are not ethernet frames they can be HDLC or PPP frames but not ethernet.
Even if doing ethernet over SDH you would have ethernet frames carried inside HDLC or PPP frames and someone should extract them to send them out the destination port.
the configuration guide says:
Except in PFC3C mode or PFC3CXL mode, Ethernet over MultiProtocol Label Switching (EoMPLS) ports cannot be SPAN sources. (CSCed51245)
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/span.html#wp1079189
but you shouldn't be able to find an example where the source interface is not a LAN interface or a Vlan (L2 broadcast domain= a collection of LAN ports).
in sw based routers ISR and C7200 have been introduced recently the embedded packet capture feature but this is not available on C6500.
Hope to help
Giuseppe