Just an update to show how I finally solved it. Thanks again zerocool for pointing me in the right direction!
/*
... output omitted...
*/
script UpgradeIOSImage
(
NCM.Nodes @ContextNode,
string @IOSImage
)
{
string @URL='scp://username:password@ip-address:' + @IOSImage
CLI
{
copy @URL flash:@IOSImage
}
}