I know it can be done a few ways through portals, layouts and value lists but I'd like a function that will cleanly and simply access an arbitrary related record by number.

Something like the getRepetition(repeatingField;number) function or the repeatingField[number] whatever-it-is. Something like:

getRelatedFieldNumber(relationship::fieldName;number)

This would be particularly useful in loops but probably useful elsewhere as well.

Use the Relationships Graph to diagram the relation between the primary key and foreign key. Then just use the lookup function.

I'm intrigued. How would you do this in a loop? I would want the value from the first related record, then from the second related record etc. I'm using these values as parameters for a script, not filling in a field.