Martin, I’m not sure what I wrote was saved, so am posting again.
After 12 years of writing VB code and creating complex formulas for MS Project, I have found no way no way to access the description column of an Outline Code field using a formula in a custom field. I know of 2 ways to access the description information.
1.) When you group on an outline code field, the description in the lookup table displays as the Name in the virtual Group-by-Summary tasks, instead of the value that displays in the field. From there, you can copy the Group-by-Summary task names and paste them into another field – it’s manual, but if you need only to populate the field once, and update rarely, it beats copying and pasting the description directly from the lookup table.
2.) MS Project provides access to the lookup table through Visual Basic for Applications, which is built in to all MS Office applications. It is possible to write a macro that can read the value in the outline code field and write the description in another field. This kind of process would execute quickly. You could add a ribbon or tool bar button to make the macro easy to run on demand. It is also possible using VB to capture a newly selected value and auto-populate another field with the description as values are selected, but this requires considerably more effort to pull off. All of this can be done using just MS Project. No other sofware is needed.
If you still need to resolve this "opportunity for success", a copy and paste in a grouped view isn’t adequate, and a simple macro that populates a field with the description at the push of a button is sufficient, I can provide you the basic info you need to make it work. Feel free to write me at firstname.lastname@example.org. Good luck. Gary Phillips, Integrated Master Scheduler, PMP