Hi,
we have the same problem and it seems that there is no out of the box kind of solution.
If you are using an xml based engine like us and the value for the keyfield is within the source data you can get the data from /aif/pers_xml by matching the MSGGUID from the index table. Then you can just do a Transformation to get the struct containing the original source data.
the code would be:
call transformation id source [data from /aif/pers_xml] result root = <FS for the Proper Struct Type>.
This field symbol can then be used to update the keyfield.
If the keyfield is within the target data i fear you have to somehow apply the transform functionality available in /Aif/err. I tried to do a quick debug session to find the exact function modules or classes you have to use but didn't find anything usable yet. Perhaps you are luckier than me 
Kind Regards.
David Rittinghaus