Hi Eric,
This is an interesting one.
Modifying value string might be an option but in my opinion is quite a serious "brain surgery" on the system :-).
You may want to try another option.
In SPRO -> Controlling -> General Controlling -> Organization -> Maintain Versions
for version 0, Controlling Area Settings, Settings for Each Fiscal Year in tab Price Calculation there is field Revaluation.
One of the options there is 'Own business transaction'. With this option posting to sending Cost Centre resulting from actual activity price revaluation will be posted with different Business Transaction (although same secondary cost element as original activity allocation).
In reports you can separate postings based on Business transaction.
It definitely works for Activity Update 1 (revaluation of production orders).
I'm not sure if it works for Activity Udate 2 (no revaluation of production orders).
Please let us know the results of your investigation.
Regards,
Szymon