Hello mmelbourne,
You asked"When a service-policy is attached to an interface, for example, a policy which defines an outbound QoS policy, is the global-service-policy still in effect"
No
"Does an interface-specific policy override the global policy?"
Yes
From the command reference
If an interface name is specified, the policy-map only applies to the interface. The interface name is defined in the nameif command, and an interface policy-map overrides a global policy-map. Only one policy-map is allowed per interface.
Only one global policy is allowed.
service-policy
http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_70/cref_txt/s.htm#wp1498207
Hope that helps! If so, please rate.
Thanks,
hemendoz