Is there a way to get around CORS/cross domain issues when building custom services on the SCA site to be called from an external page?

This isn't a supported use case. Commerce services are designed only to be consumed from within the context of a NetSuite web store

Great, thanks for the information!

If your SuiteScript is good you may want to look at Suitelets and RESTlets and other types of integration opportunities (downside is that they won't have the commerce 'context')