How to get value of a Relate field for use in calculate field


I have a custom module that I'd like to automate the name by using good old calculated field Concat feature.

Problem is that one of the values I need to concatenate is in a Relate field pointing to Product catalogue.

I am unable to figure out how to get this value, as it doesn't seem to be exposed for us to use it.

Any ideas, or is this simply a missing feature?

Thanks in advance.