Hi Carpsy,
You can get the SPECIFICATION of pipeline and populate that in SPEC property of ISO sheet using the EC expression you mentioned, there shouldn't be any issue with that. What i am not clear is that why are you using the parser regular expression at that point? If all you are getting is the specification name, you don't need to parse it further unless you are trying to get the specification from pipeline name which can be like
Unit-Service-LNum123-SpecName.
If you are reading SPECIFICATION property from pipeline, it would be same for all associated ISO sheets. Changing it on pipeline would result in updating all ISO sheets. So i think you would be manually updating the ISO sheet SPEC property for that specific instance using ISO sheet manager.
Next would be to update the ISO sheet name ... which would require a little extra effort but can be achieved (if thats what you mean by "This way moving forward when you generate the iso sheet you do not manually have to change the drawing each time").
If you can elaborate a little on what do you want to achieve, i might be able to offer a better solution.